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Been surfing the net with no luck trying to find plans for a diy switching box that would allow me to have two amps fired up and switch between them to one set of speakers. I imagine that the one that isn't connected to the speakers would have to be connected to a dummy load and that "break before make" switches would have to be used. So far I have not found anything and am hoping some here may know of something. I want to use it to A/B two amps.
The other thought I had was to put resistor loads across each set of speaker terminals on each amp so that they would always have a load and then I could just use a simple selector switch. The thing I don't know is how that would affect the sound. How little resistance could I get away with and still not hurt the amp? Would even 2 ohms change the sound of the speaker?
I listened to both amps for several hours last night and a couple of hours this morning with all different kinds of music. All I can say is that with the volumes carefully balanced I could not hear any difference between them except a little stronger bass on the 6bq5 amp. When I pushed the button to switch amps, if I had not seen myself make the change I would not have known it had been made. There was no audible difference. This was not what I had expected when I set up this experiment, but it does match the results I got one other time with two different amps. Not exactly the same setup, but the same principle. I kind of think it is the fact that they are both SE amps that is the main reason.
I have also read in several places that in blind testing of amps most people can not tell one amp from another. Going to have to test that theory, so I am going to build one of these 6bq5 amps to use in my system and dismantle the 813 for parts. Then I will build a simple PP amp (6v6 maybe?) and do the same test between the two of them and see what happens. If I don't hear any difference then I think I will be upset, because it will mean I have been chasing a rainbow all these years. Then again, maybe not, because it sure has been a lot of fun.
Just decided I really should do the same A/B between the 6bq5 and my 26/26/45 all dht amp and see what I hear before I do anything else. I will report back after I have done that test.
On a positive note kudos to you for actually building and listening. You are correct that most music is no where near 12k. That being said I would guess there are some issues somewhere in the chain to not hear differences in the 2 amps. We are not talking 6 inches of wire.
The good news is if you keep building and comparing you should be in for a real treat. This use to be a forum where people would break from the status quo and report what they heard. Next on your list transformer coupled/split push pull.
Enjoy the ride
Could you expand upon "That being said I would guess there are some issues somewhere in the chain to not hear differences in the 2 amps"? Quite frankly I am looking for an explanation. Especially now as I just got through doing an A/B with the 6bq5 and the 45 and got the same results as the A/B between the 6bq5 and the 813. The 45 is all dht. The 6bq5 has a triode driver and a pentode output mode. Both are SE.
Let me state the test conditions
1. Only one set of speakers was used, my Altec Granadas to be exact.
2. Very careful adjustments of volume levels were made before the test began; there was no audible difference in volume when switching between the two amps.
3.Only one source of signal was used, a direct feed from my cd player that used a pair of y-splitters to take the signals to each amp. Both amps were on full time.
4. A switching unit was used that had both amps as inputs and one output to the speakers.
5. A load resistor of 10 ohms/30 watts was placed across each pair of speaker terminals in addition to the speaker connections to make sure the amps could be switched hot without damaging them.
I also used the same set up yesterday when A/B testing the 6bq5 against the 813.
In all cases there was no audible difference to my ears between either pair of amps in either test, except that the 6bq5 seemed to have a tad bit better base. It may be my old ears are not hearing the higher end of the the scale, but if you look at the links below in my reply to cpotl's post, you can see that that still leaves a lot of music that I should be able to hear.
I read in the one link on "do amps sound the same" that one of the biggest factors in them sounding different is distortion, and in particular which harmonic it is in. Anyone used to SE amps is most likely familiar with the fact that the distortion from a SE amp is less disturbing than from a push pull amp because of this. To me it seems reasonable to think that if this is one of the factors in amps sounding different, if all the amps have the same distortion by being SE, then they might sound very similar in an A/B test.
To test this theory, my plan is to next build a simple diy pp 6bq5 amp and repeat the test between the 6bq5 se and the pp amps and see what happens.
In the meantime, if you know of any other contributing factor for me to check out I would appreciate hearing about it.
Vinnie I was thinking about your quest last night. There are so many variables known and unknown. If I remember correctly you could hear the difference between the coffin white resistors and mills in one of your builds. This is a good sign. The age over 12k thing is, let's say, not relevant.
The mystery to me is using the same amp and just rolling tubes can cause big differences. You have two different amps with different tubes including dht which have there own sound. And you are not hearing a difference. Are you using same brand resistors caps etc. Maybe they could mess the end result by blocking potential??? Do you have a analogue front end that you could play around with.
I was thinking if can afford interstage transformers, to grab a couple for your push pull project. This will kill at least 3 birds with one stone. You lose the plate resistor, coupling cap and some say the best way to split the signal for push pull. Build a simple 2 stage and listen. You can 3 stage etc latter as you keep building and listening.
A clue on one thing dht do so well at is listen to a chorus listen for each individual voice as well as the harmony. It really brings home there are various singers and combined create harmony. Never hear anyone talk about that instead the black background and placement which I never have heard in a live venue. I am not trying to influence your subjective preferences just maybe help you in hearing some sonic differences.
Just finished up with a permanent placement of switched 12 cap panel per channel on my let say extremely large LCR transformer coupled phono stage. I had it external but the sound was different with each brand of cap and depending on which lp, I had different preferences. So with no board of directors or bean counter to appease, let's take up some more real estate. These caps are not even in series and yet sound quite different in some cases. So the moral of the story is everything makes a difference.
Just thought of another tip for comparison listening. Say you know a certain passage on various albums. Could be to hot or cold. Maybe a drum solo with rim shots or vocal with real body. Then just listen closely for your examples you know
Inside and out.
Vinnie you are in the best position to build the very best. You do not have to show a profit or make it small etc. You can mix and match and start over with no dead lines etc. Build and enjoy, the soul of the music is not over 12k.
Enjoy the ride
From what I have read, some people think that one of the biggest differences in amplifiers is caused by distortion. I want to investigate that before I do anything else. I have picked out a nice little schematic for a simple pp 6bq5 that I will be bread boarding in the not too distant future I hope. If I don't hear enough difference between the SE and PP amps I will have to start looking at other things.
I am already doing what you suggest about listening to familiar passages. I have several cd's with tracks on them I know very well, and that is what I listen to when comparing amps. The tracks includes various instrumental selections as well as some vocals. Some are lively and some are less so. I think I am testing with a fairly broad range of music including jazz, classical and pop.
I couldn't agree more about having an advantage for building, but it is also important to try and build the best amp I can for the least amount of money. It's a challenge I enjoy. I have mentioned in other posts how I do not believe in spending mega bucks to chase what I call the last 5% of the asymptotic sound quality curve because you very quickly reach a point of diminishing returns. You have to make trade offs sometimes, but that is to be expected in any project.
Sorry to say this, but you're too old to make comparative amplifier judgements based on audible evaluation. You'll need to call in younger friends or family members for this if you really want to know whether there's a difference. Nothing personal, happens to us all.
Buy Chinese. Bury freedom.
Sorry to disagree, but for most recorded music it's not a problem because it's more toward the middle and my judgements will be valid. Most older men are usually good to 12k, and that covers a lot music. I won't be listening to test signals.
"That's true for anyone over 45 if you are talking about the high and low extremes, which the average person never listens to anyway. For most music it's not a problem, and my judgements will be valid for the most part . I won't be listening to test signals."
You're rationalizing. If you can't hear it, you can't hear it. In fact, your inability to hear the highs and lows is probably the reason you're OK using PA-quality output transformers. This - and your admitted inability to hear differences between various amplifier topologies - totally undermines everything you just said.
Buy Chinese. Bury freedom.
Oh for pete's sake TK, give it a rest already! Why don't you go build something so you won't be harping at everyone else all the time. You are beginning to sound like a broken record. When was the last time you posted an ongoing amp or preamp project with pictures? Don't you think it's about time?
Vinnie, I admire your spirit and optimism!
I started playing with tubes and electronics as a teen in the 1960s, so I'm firmly into the age range TK mentioned, where the higher frequencies, at least, are fading. And he is quite right, I have to say. But although I've already built N amplifiers it doesn't stop me from wanting to build the (N+1)th, even if I can't hear any difference.
In the end, the things I enjoy are the music itself, and at the other end of the spectrum, I can enjoy tinkering with the circuit to boost the technical measured performance, even if I can't actually hear the difference.
Actually, I'm not sure I would ever have been able to make trustworthy judgements of the difference in sound between systems. I learned recently I have aphantasia (no mental visual imagery), and I am suspecting I have the audio analogue too. It has always been a mystery to me how someone can compare two sounds that they hear sequentially.
I do enjoy the hobby, that is for sure. As to the hearing part, I am not so sure he is all that much correct. Take a look at the attached link and you will see there is an awful lot of music below the 12k mark, with the ear being most sensitive to the range of 300 hz to 5000 hz. Even us old farts can still hear a lot of recorded music and therefore hear differences if they are there.
Soooooooo, the next obvious question is, is there really that much difference between amps, and if there is what else besides frequency range might be at work there? I am about to start a web search on that topic and will report back on what if find.
Are you testing the amplifiers at all listening levels? I find that many amps do sound similar or the same at low levels but when pushed the shortcomings are apparent. I start my testing with something dynamic.
These shortcomings can almost always be measured so you need to challenge yourself by using a scope more often and do some frequency sweeps. More advanced hobbyists may use equipment to check distortion.
Can I hear a difference between a Hammond OPT and a Slagle or Monolith OPT? The answer is hell yes and they all measure different as well.
I read that most amps start distorting a little below clipping, but I rarely push mine that hard.
My philosophy for mine own listening is that if I can't hear a difference, then for all practical purposes there is no difference. I am not much of a fan of dialing in the last little bit with instruments if I am not going to be able to hear a difference.
I have heard differences between some opts also, but even the late Don Garber said the 125ese "sounds better than some people would like to admit" and offered it as an option on some of his builds.
Right now I plan to focus on building a small diy 6bq5 PP amp so I can do the A/B test between it and the SE 6bq5. I am guessing I will hear a difference between them. If I don't, then I will have to start digging deeper for an explanation. I am determined to find one.
"My philosophy for mine own listening is that if I can't hear a difference, then for all practical purposes there is no difference. I am not much of a fan of dialing in the last little bit with instruments if I am not going to be able to hear a difference."
That's fine, but it's not what we're discussing. Here's what you said earlier that started all this: "I kind of think it is the fact that they are both SE amps that is the main reason." That statement means you believe the lack of audible difference lies with the equipment, rather than your own lack of auditory acuity. This is what I was responding to.
Seriously, I don't understand why you're arguing about this. It's not rational to believe we can hear all the subtle differences when we get older. I was only making a comment that I thought would be helpful.
Buy Chinese. Bury freedom.
Tk most of your responses to my last several posts up to now have been both antagonistic and condescending to say the least. I don't mind helpful suggestions or even pointing out errors in a friendly way, and you have done both in the past. What I do mind is the attitude that you have once in awhile that I am an idiot if I don't see things the way you do. If you don't agree with what I say, fine, you don't have to, but please temper your replies with a bit of restraint.
My comments were based on the links I posted, and there is some very valid information in them, including how much of recorded music is below 12k. Look at the chart and you will see what I mean. I am attempting to understand why I am hearing what I am, and I plan on taking it one step at a time. Any suggestions you can offer in a friendly manner that might help me figure it out will be welcome. I think there is more to it than just me being an old fart or using less than mega buck opts. I plan to report my findings here as I go along.
Vinnie, I don't think TK was being condescending, sometimes it's difficult to interpret the tone of an email or post. This new digital world can be challenging at times.... Engineers deal with facts, statistics and measurement data, sometimes answers may seem harsh but i never take it personal. I work with electrical engineers every day, I always back up my answers with factual data. We all know music is under 12k but I'm sure there's a reason why we choose transformers with extended bandwidth, sometimes 100khz and above.
My Hammond opt performed pretty darn well, especially in the base region but didn't have that live sound of other opts. Could it be the extended freq response of my other opts? Or the amorphous core? I just don't know the answer to that but do hear a difference.
My Khorns reveal everything and makes this process easier to interpret.
On my ribbons, not so much.
" We all know music is under 12k but I'm sure there's a reason why we choose transformers with extended bandwidth, sometimes 100khz and above."
If we look at a simple, single pole 6db per octave low pass filter we see that the amplitude is still down 1db one full octave below the -3db point.
A OPT that is rated for -3db at 15kHz will still be down 1db at 7.5kHz
Then there's the matter of phase shift. A filter causes not only a loss of amplitude but also a shifting of phase. The music's phase is shifted for a decade from the -3db point. So if the amplitude is -3db at 15kHz the phase will be shifted all the way down to 1.5Khz
If we want the response (phase and amplitude) of a system to be flat 20Hz-20kHz we need the filter's (high pass and low pass) -3db points to be 2Hz-200kHz.
If we want response of a system to be flat 50Hz-15kHz we need the filter's (high pass and low pass) -3db points to be 5Hz-150kHz.
These are "tall" orders, especially for an output transformer, but there are real world benefits in meeting them whenever possible.
BTW There are low pass and high pass filter everywhere in a circuit. Every cathode bypass cap forms a filter, every coupling cap forms a filter. The Miller capacitance of each stage forms a filter. Stray inductance, stray capacitance etc. It's not just OPT's.
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm sorry deaf, but if it is that easy to misunderstand the tone of a given post, then the poster needs to be more careful. We are talking about being polite here, and being technically inclined does not excuse someone from politeness.
As far as the rest of your post, I am simply trying to show that there may be more involved with this than just opts and old age. I will keep looking to see what turns up.
what are your operating points for each stage of the amps?
It would seem there is more at work than just frequency response
I would imagine that some of those horrendously bright speakers I dismissed in the 1970s would sound pretty good to me right about now.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Well I got it all put together as you can see in the photos. The resistors (10 ohms/30 watts) on each set of speaker terminals seem to work ok. I played them both for about half an hour this evening and did not encounter any problems. I used a rat shack switcher unit with the output going to my altec granadas. Just have to push one button to switch between them; both amps are on all full time.
Now comes the really interesting part... I listened to some blues and some jazz (will listen to pop and classical tomorrow) and I really could not hear any difference between the amps on any of the music I played. I was very careful to balance the volumes very closely before I started comparing them so that it was not a factor. This is not the first time I have had something like this happen. I really think the fact that they are both single ended has more bearing on their sound than the specific tube, even though the 813 is straight triode and the 6bq5 is triode for the driver and pentode for the output. The other interesting thing is the 813 amp has magnequest fs 030 opts in it and the 6bq5 only has 125 ese's. Makes you wonder how good it would sound with a really good set of opts. That would be an interesting experiment, and one I may have to try (like with the fs 030 opts maybe?). Any way it has me thinking about building a finished 6bq5 for my main amp and using the 813 parts for other experiments. I will be listening to a lot more music before I decide on that, but I never expected it to be so close right out of the box.
I know who I am calling when I need some tubes!!!
Great shop and it is obvious you are enjoying it.
One of my earliest projects was a 'taste tester' line stage where I could quickly switch between various tube types. It was an interesting project and the details are in the link. However the results were what I think you need to be aware of. Basically, your amp preference may depend on the music you are playing.
And, no, I do not have a cunning solution to your stated problem.
So the big question is whether you can actually hear differences between the various tubes. The answer is yes, which is not surprising. When I first started to get music to appear at the loudspeakers I quickly shuffled between the various tubes and immediately noticed differences. Gain levels vary, of course, so I needed to adjust the volume to do real comparisons. Actually, I found it works best to resist the temptation to flick around between the tube types. Certain tubes seemed to suit certain music and I seemed to be consistent in my tastes. If I put this in whisky terms, if it's really cold, wet and miserable outside, I prefer the peaty Islay types like Lagluvin and Laphroag. Otherwise if it's just plain wet, then Dalwhinnie works for me. If it's dry, then I prefer Ezra Brooks bourbon. If it's after dinner, then Pierre Ferrand cognac is the tipple of choice. Variation according to your needs is wonderful.
Interesting post. My actual objective with this is to compare two (for now) of my diy amps with A/B testing to see which one I like the best. Some say you can't tell differences by switching back and forth, but from my experience it is the best way because of the shortness of sound memory. I think the trick with the resistors might work, at least it is worth a try.
I think that what you meant to say is 'make before break'. IOW make the connection to the dummy load before disconnecting amp A. For a brief moment the amp will be seeing around half the impedance, which beats the alternative of the OPT becoming a flyback.
Make before break with tube amps. I have some old early 1970s Lafayette Radio catalogs. They sold simple speaker selectors and there were two versions of each model listed. Tube or Solid State. The tube units were make before break and the SS units were break before make.
This is not a casual difference. A tube amp running unloaded for even a few hundred microseconds is enough to arc an output tube. Likewise two solid state amps connected in parallel again for even a few microseconds can destroy the output transistors.
That sounds reasonable, but I will have to check the article I was reading about switches again because I thought it had to the other way to prevent a short between the two amps. I think I will probably go with the resistors and that will make it a mote point. Safer that way.
If you put a 10 ohm resistor across the speaker terminals then, as you say, the amplifier will be seeing a 4.4 ohm load, instead of 8 ohms, if your speaker is 8 ohms. This is a very significant change to the loading.
I would have thought you would want a shunt resistor that was considerably higher than 10 ohms. Essentially, you would want "as high as you can get away with" while still achieving the goal of keeping the amplifier OK in the event that it is running without the speaker load. 10 ohms would certainly be OK from that point of view, since it is barely more than the 8 ohms of the normal speaker load. I have no experience of how high you can get away with, but it must be a good deal higher than 10, I would think.
I've not thought through the details, but I would think an "ideal" arrangement would be one where there is no resistor shunting the amplifier when it is the one selected for listening. A make-before-break switch is needed for switching the amplifier either to the speaker, or to the substitute resistive load. And arrange, maybe by using a multi-gang 3-way switch, so that you can do this for each amplifier.
Assuming that you don't use permanent shunting resistive loads, you need to use make-before-break switching so that an amplifier never operates with no load. But you also need to make sure that you don't end up with an intermediate switching state where the two amplifier's outputs are connected together, since in principle there exists the possibility that the amplifiers are in antiphase (one inverting, the other non-inverting) and so connecting their outputs would be like a dead short. I think some arrangement with (maybe ganged) 3-way make-before-break switches would achieve all this.
It must be a standard problem, with a standard solution, I imagine. There should be schematics floating around on the internet somewhere. Otherwise, one can work it out with bit of thought, keeing in mind the two essentials: no amplifier ever open load, and the two amplifiers never connected together.
Just enough to keep the transformer primary impedance to a safe level.
Your last sentence is exactly why I think I will try the resistors on each amp. The old KISS principle once again. With the resistors there is no chance of damaging equipment with a dead short or no load. At worst the sound will be funky, but I ought to be able to notice that. If you happen to run across any plans please let me know as my searches have come up empty for a unit that will allow switching with both amps running.
Sorry for the poor image quality, but attached is a sketch of the sort of thing I had in mind. Two ganged 3-way Make-Before-Break switches, wired as shown. I've had a couple of beers (in China right now, where it is evening), but I think it does the job of ensuring neither amplifier ever operates with no load, and that the two outputs from the A and B amps can never be connected together as the switching occurs. The indicated positions of the switches are for Amp. A playing through the speaker. The middle position is neither A nor B playing through the speaker. R would be 8 ohms or so. Of course another 2 gangs are needed for the other channel of a stereo amplifier.
The schematic assumes it is allowable to have a common ground for the outputs from amplifiers A and B. If this is not allowable (e.g. circlotron OTL amplifiers) then I suppose it would need a similar switching arrangement for the other end of the speaker too.
I could imagine there are more efficient ways to achieve the desired goals, but I think this would achieve it (unless the beer was too much!).
I appreciate the effort, but I tried just adding a 10 ohm/30 watt resistor to each set of speaker terminals on the 6bq5 amp and played it that way for an hour or so. Did not notice any major difference in the sound from without the resistor.
Going to have to put the amps side by side with the resistors mounted and then use a regular switch unit between them to go back and forth. Just too much trouble to route all the wires that would be needed to go to a switch from the equipment rack.
I'll do a follow up post when I get everything hooked up and working.
I found a program that calculates impedance for speakers in parallel (see link). According to it if I put a 10 ohm resistor on each pair of speaker terminals on each amp, when an amp is connected to the speakers the impedance will be 4.4 ohms. These are 30 watt skookum resistors so they should handle the load on idle. This would seem to be the easiest and least expensive solution to do some A/B comparisons. I know the resulting impedance isn't a perfect match, but it ought to work for an A/B test I hope. Does anyone see major flaws with this?
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