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Newbie Alert. Just wanted to say that up front so you would know who you are dealing with before responding.I can only use the first 6 detents out of 40 with my 100K pot in my preamp. I have seen numerous post on this forum saying the same thing. I have not found a good "fix" in any of the post. Most took the preamp back and tried a different one. My preamp is 10 years old so I do not have this option. (Also I am loving messing with it!) I think my pot already has audio taper but it still is hair-trigger. It having detents only makes it worse.
I have tried with the help of John Curl to put a 100K resistor in series with my existing pot but my preamp does not seem to like this. I think it needs to see 100K impedance to work properly but am not sure. See reference post here:
http://www.audioasylum.com/scripts/t.pl?f=tweaks&m=57947
Is there anything else I can try other than one resistor in series with my current 100K pot to achieve my goal but not change its 100K impedance or creat too much of a filter?
Does anyone make a 100K volume pot that spreads the first 2% to 6% of the range over the entire turn of the knob?
If not can you use something like a 5K pot and add resistors to it to make a 100K pot that will do this? If so I would need exact details on resistor values and were to attach them to the pot. Also I would need a source for a good 5K pot.
Any advice will be appreciated,
Take care,
Ronnie Ferrell
Follow Ups:
Take the 100K resistor and rout it from signal to ground. (You're just made a voltage divider and dumped some of the signal from the preamp. You've also dropped the input impedance of the amp, so your pre might not like the extra current demand. If it doesn't not, put another resistor in series with the signal. Play with the value until you like what you hear.(If someone already suggested this, sorry to waste the bandwidth.)
Ok, here's what you do. Give Jensen Transformers a call and order up a PC-2RR IsoMax and install it at the input of your amplifier. The PC-2RR uses a pair of 4:1 stepdown transformers and will attenuate the signal by 12dB. You'll also get the benefits of a good quality input transformer such as elimination of ground loop and interchassis currents, improved common-mode noise rejection, etc.At a little over $200, it's not a cheap solution however. And if I'm not mistaken, Jensen has a return policy so you can return it if it doesn't work well for you.
Another and less expensive solution would be to find out why, when you bypassed the input opamps in your power amp, that the amp's voltage gain increased so dramatically that you can now only use the first couple of clicks on your volume control. I suspect that when doing the mod you also somehow altered the amplifier's feedback network.
se
thanks,ja in its getting too hot too soon, Tucson, too dry, AZ
The primary downside from a technical standpoint (and applies to virtually every transformer) are the nonlinearities due to the core material which are largely confined to the lowest frequencies. There can also be a downside if you've got significant amounts of output offset voltage or input bias current in the source and load components respectively. Other than that, a good quality transformer is pretty faultless.se
Is it a log pot (not a linear) and is it wired correctly so that the taper is slow at the bottom end?
Yes, it is a log pot (audio taper?) I think? I have checked it with a MM. 50K to + and 50K to ground is at about 80% or 90% of the full rotation of the pot or something close. I know it is not at the halfway point.I have the values at 6 detents written down at home. This is the values I am looking for at about the 12 O'clock mark. I just don't know how to go about getting it.
Thanks,rf
Then the problem is too much gain. This can be solved as JC suggested with a series resistance or with other adjustments.
I tried that but I think something in my preamp does not like the impedance change (just a guess). It has a greater noise floor after I put the resistor in before the pot and 100k only gives me one more detent of control. 470K in series gives me the control I am looking for but with greater noise floor and Jon said this is way too high of a value to use in series with the pot.I also here the audio signal in the tweeter with the volume all the way down after I put the 100k resistor in series. It is dead silent without the resistor.
If I put the 100K resistor in series after the pot I do not get the greater noise floor, but I think I can here it filtering the top end some. Also I still only gain one more detent on the pot for a total of 7 out of 40. Gaining only one more detent does not out way the effects it has on the sound.
Is there a way to lower the gain but keep the 100k impedance? Can I get this with 2 resistors as in a T-pad or L-pad without filtering something out too much? If so what value of resistors and where do I put them?
How about lowering the gain of the amp, itself, rather than attenuating the signal?
This all started when I said, "My equipment is 10 years old. Its paid for. Why not try to tweak it and learn something at the same time." If I make it better then great. If I screw it up, I go buy a tube amp and preamp. I took JC's advise and bypassed the op amp in the amp. WOW! This made a big improvement in sound quality! Enough for me to keep my equipment for a while longer. But after this the sensitivity of the amp increased dramatically. I can live with it but if I can fix it then great!My initial thought was, "can I adjust the gain internally" since it was not an issue before bypassing the op amp. I asked JC and he was against doing it internally because the amp may become unstable. I am taking his word as law on this one since he designed the amp in the first place and I am a peon who knows jack squat. [but I am learning ;) ]
Then I asked if I could put an attenuator in the male RCA's right before it enters the amp. See this post:
http://www.audioasylum.com/audio/tweaks/messages/57947.html
That is when I got the suggestion of changing the attenuation of the preamp volume pot:http://www.audioasylum.com/audio/tweaks/messages/57948.html
I do not know the output impedance of my preamp or the input impedance of my amp, or if it even matters. So even if I go this route I do not know what resistors to use.
Thanks again for taking the time to help a newbie learner!
rf
I reduced the sensitivity of my amp by putting in a fixed resistor voltage divider at its input. Worked well for me. This is pretty simple, you put 2 resistors between the RCA pin and ground, and the cable going to the amp attaches to the middle of those 2 resistors, instead of to the RCA pin like I did before. You can vary the values of the 2 resistors to control how much attenuation you want, and how much input impedance you want your amp to have. 2 linear equations, 2 variables, easily solvable :)
This will work.
RF, you are making a mountain out of a mole-hill. You do have too much gain. This is obvious. At most, you have increased the overall gain by 6dB, by taking out the input pot. If you have even more gain, then there is a problem with what you did to the amp. It is possible, in principle, reduce either the gain of the power amp or the line stage of the preamp. You could also reduce the gain of your source. Since, you are complaining about noise, then you have too much gain in either your line amp, or your power amp. It has little or nothing to do with the pot, as the pot is almost shorted out. Steve Eddy's transformer solution would actually work well in this case, but it is not absolutely necessary. You could put a 50K resistor across the wipers of the volume control to ground This would reduce the gain of the pot, to some degree, but it is not a perfect solution. The best overall solution would be to replace the volume pot with a lower value (25K or so) and use a series resistor to reduce the input sensitivity (75K or so) and get the 100K input resistance back. The alternative is to reduce the preamp gain at least 6 dB by reducing the value of the feedback resistor by 2, although this might make the line stage oscillate. This is why we have test equipment available, when we experiment.
Also, you could just build an attenuator between the preamp and power amp, just like you suggested initially. Just put it near the amp input, so as to not roll off the high frequency response. This preamp should be able to drive a 20K or so input resistance. The output cap might roll off a little too early, but you can change that if you want.
Sorry. I do not consider this a mountain or a mole-hill. I considered it a learning experience. If newbies are not welcome here with what may be stupid, idiotic questions to the experienced, what good is a public forum.Thanks anyway for what I have learned so far.
Take care,Ronnie Ferrell
What I meant, was to just relax a little, and remove yourself from the problem momentarily. You are making the problem more complicated, than it is in reality. Normally, you should not have to do anything to change the gain of the system. My associate, Carl Thomsen, the T in CTC, uses an HCA-2200 modified by me, with the CTC preamp that I also use with a modified HCA-3500. Neither one of us has any noise or gain problem, so it is difficult to pin down what your problem is. Are you using really efficient loudspeakers, like over 100dB/W ? This could make a difference. We use speakers in the low 90's.
Did you accidentally change the gain of the power amp to a much higher value? You should try to check this with an ac meter and a test tone. Do you have any test equipment? Much of what is confusing is not because anything serious is wrong, but that we don't know the state of the equipment under test. This leads to a lot of idle speculation and hand-wringing. It is best to look at each piece of equipment, one at a time. Good luck, and keep us informed.
I am taking a break from this right now, the weather is too nice here today to be inside too long anyway!J Epstein web link below is helping me sort out some of my basic confusion.
I do have a test CD with a 1kHz test tone track. And I have a cheap (very cheap) MM. Right now if I do a test to post my results, I will need someone to explain in layman's terms or have explicit directions to put x here and y here set this to that and tell me what you get. Just realize this before posting if you want to continue...
rf
John has been very forthcoming in his suggestions and does not merit your response. (I backed out when it became clear that I hadn't the time/patience to help.) I see nothing insulting or demeaning in his statements.
I apologize to John personally and to the forum for that last post.I am more frustrated with myself and my lack of electrical knowledge than anything. I know I am over my head with the most basic of questions. I need more basic education before asking my questions and I am realizing this. I need to back up and find a good book that explains the very basic concepts like input impedance vs. output impedance.
I cannot believe John has helped me as much as he has. And he keeps posting to my questions no matter how trivial. I know I would not have help me this much if I was in his shoes. I almost passed out when the actual guy that designed my amp posted to my first question. I could not believe it.
I am truly sorry and please forgive me for taking my own frustration with myself out on John and the pubic form. I know this is a great public form and find it invaluable. And John I cannot thank you enough.
Ronnie Ferrell
ps- is there a "DIY amp/preamp book for dummies?" One that explains basic terms, components and sections of an amp/preamp.
http://www.electronics-tutorials.com/basics/basic-electronics.htm
.
It takes a good attitude to apologize, so you are on the right track.We've all been wehre you are and felt a little overwhelmed. Take a break for a day and come back fresh.
I'll think about your "what book?" question, see what I can come up with.
Don't worry, you'll get where you want to be, it's not rocket science.
If it's any consolation I found input & output impedance very confusing too.
-j
nt
I, of course, meant input IC, not input pot.
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