|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
50.132.29.220
In Reply to: RE: SUN SV 2A3 stereo SE amp .... MEASUREMENTS posted by elblanco on April 01, 2015 at 10:58:33
That might be too technical for Jeff. (The middle red dot is the quiescent operating point)
Making these changes, you can expect less power and higher THD. I'd expect just over a Watt RMS before things fall apart. Jeff likes to talk about overload recovery, but when you stress an amp like this (which will happen on GPA-604's), you will drive the amp to the point where the 2A3 will be off on one side of the waveform.
Heading in this direction brings you to class B amplification, which is useless without some feedback to keep the distortion under control at useful power.
(yes, I have heard these amps)
Follow Ups:
Class B SE. Thats a new one.
here's my most recent measurement after I got up and running again.
I"m a bit off from Jeff's ideal... i'll adjust later... 270v @ 46ma
12ax7 driver, 285k plate loads, this measurement was with a 30uf cathode bypass cap on the 2a3. have not measured since going to 10uf dc-link.
both measurents at 1 watt.
class b, doesn't work, horribly distorted.. blah blah blah
the cree schotky diodes are keepers too, although jeff wouldn't agree!
-5dB down at 20kHz ? Not hifi , not even midfi but I suppose some may like this sort of thing . Either this is deliberately poorly-engineered amp or you are using crap output transformers . I can't think of any such causes in a properly engineered design , it certainly should not be the driver stage causing this rolloff...
I think it's time for you to find your own way and put this Lowmu/Dennis crap behind you .
Al
Rage, thanks for posting the measurements.That's quite a bit of HF roll off. It seems to me that would be audible in a way that would have a negative impact on the sound.
My educated guess as to the cause of the roll off would be the inability of the driver stage to properly drive the Miller capacitance of the 2a3.
Isn't there a driver tube that can give you the gain you require, while at the same time be able to fully drive the Miller?
From a technical POV, I would think that would give a "truer" sound.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/12/15
let me dig up the measurements with 6ej7... hf rolloff wasn't that much better, but maybe I read it wrong.
both measurements with ds-025 outputs so yes definitelty circuit specific rolloff.
we'll see how I like the other extreme as I"m building the KT120 PP amp with Hammond 1650R outputs right now... in a 12u rack chassis on casters. :)
Is it really -5dB or so at 20Khz?
dave
Thank you for measuring and posting this Rage !!We "all" KNOW that a 12AX7 at .0006 A. can't drive a 2A3's Miller capacitance, in this Direct Coupled amp of yours. Some "theoreticians" who never heard our amps have REPEATEDLY told us that up here.
Surprise surprise when you measure it.... or HEAR it, .....and its way cool.
Thanks again, nice work. Crees will be low Z, but then there is long term start-up considerations with a solid state rectifier in a tube amp, unless you switch filaments "on" first, and delay the B+ high voltage.
Have fun, I sure am !!
Jeff Medwin
Edits: 04/11/15
Then walk us through it ...because what I built from you posts sounds like a dynamic expander with a healthy dose of distortion.is it inviting ear candy?, yes..is it true to the source, NO. Its OK if you like that, no problem, but its not music. Its Dr. Seuss hifi, like Sakuma san, its a SOUND. Ting tinglers and bong bonglers. I visited him once years ago and for the music he listened to it worked, FOR HIM.
I thought the distortion measurement I made was moderately low for an SE amplifier without feedback.
did you measure this amp that was a dynamics expander?
I won't deny some possible enhancement in dynamics.. I am OK with Tre's happy music machine reference in this case.
I don't hear past 15k. I realize the biggest potential issue here is phase shift given the high -3db points and lack of current to drive miller capacitance.
I'm a little familiar with audio processing and I don't hear the familier increase in density from compression and limiting. I do hear what I would call "soft clippinbg", and I think in the case of the dc2a3 that Jeff passionately presses that the compromises of the amplifier are mainly for the purpose of good sounding "clipping behavior."
a little clipping and distortion is pleasing to the ear for many, who just like their shit loud anyhow.
Still, my prefernece is studio monitors and flat response. I'd argue this amplififer is more accurate then most might assume.
Hi Rage,
You have not executed this amp quite as I suggested. You need to fully bypass the Rk of the 12AX7 driver, to extend the amp's overall band width.
A single 20 uF film Rk cap, and a 0.68 across it, will roll off the top end. You could easily pick up four more db at 20 KHZ, if you would have done a complete and linear bypassing of the 12AX7's Rk as I detailed to you in numerous emails. Its important to do.
Grant Handley, aka GSH and Satin Patiently made the same exact error. Do that, and re measure and you will see, and you will hear it also !!
Cheers,
Jeff Medwin
Updates?
If the circuit is -4dB @ 20Khz and the 12AX7 has a 20uf film Rk, there is no way any additional bypassing will miraculously give you that 4dB back.
Historically, Kondo used a very small bypass cap in the driver for the ongaku to gain back some HF lost by the output but that is doing just the opposite of what you suggest.
dave
I'll do that Jeff.
I'll be wrapping up latest build in next few weeks then can put the 2a3 amp back on the bench.
I'll also vefify my measuring setup!
His circuit is -5 dB at 20K with his not-to-my-specified bypassing.I contend, with a DC two stage amp, it is possible to get 4 dB back Dave !!!
Careful Rk multiple film cap bypassing, of not only the 12AX7 half, but also the Rk for the 2A3. NEITHER stage's Rk was built by him as was suggested, and one can not leave this out.
Allow rage to do that and re-measure his frequency response for us all.
Jeff Medwin
Edits: 04/26/15 04/26/15
I'll give you that the multiple bypass can sound different/better but on the test bench I can see no possible way that anything other than absolute value can effect a 4dB change at 20Khz. Furthermore, given the nature of capacitors, (lower impedance as frequency goes up) I see no possible way that a capacitor change can actually cause an increase in output at 20Khz compared to a normalized 1K output. What can happen by using an overly small bypass is nearly full bypassing at high frequencies transitioning to partial bypassing at lower frequencies causing what appears to be a rise in high frequency response but is actually a reduction of lower frequency output level. (Kondo-san did this on the ongaku driver to make up for loss of HF response in the OT)
In any case I'll await the bench tests....
dave
I think my Lynx one audio card is ready for the trash. Its from ~ 1999.. I see electrolytic caps I assume are on the output stage. I"m betting its responsible for some of the top end rolloff.
I've got a spare Audiosciecne PCI card at work to borrow, 96khz and fairly new.. I'll run new measurements very soon.
I am in agreement with Dave in that I don't believe the bypass caps will cause that much boost up to 20khz....and also hoping I've got quesitonable measurements. Definitely up for following Jeffs outline for educational purposes. If he's wrong I can always sell some caps or put them to use in other projects.
The sound is not an issue to me, although I do realize that there is a great deal of bias since I built the amp!
In watching a few posters on here I"ve somewhat deduced that low frequency phase response from the low current 12ax7 might be less an issue than its made to be...given the lack of global feedback.. (althouigh there is some degenerative feedback I think from smaller cathode bypass caps)
With your bench tests. Do you find frequency response and phase to be strongly correlated?
-----------------------------------------------------
Big speakers and little amps blew my mind!
I didn't think the two could be separated.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Gotcha.
--------------------------------------------------
Big speakers and little amps blew my mind!
CB,
(1) "That might be too technical for Jeff."
YOU asked for it !!!!!
FYI, I mastered graphic load line analysis in 1982, taught by the late Bob Goodman of Texas. .............Were you even born then ??
(2) You don't like "competition" I can see. That is OK. I understand.
(3) Your own graph is laughable - INCORRECT in two or three ways :
(a) You don't know the STARTING B+ level, because you don't KNOW what FILTER I am using and its effect upon B+ ( VDC level ) on the 2A3's plate.
(b) Nor do you know how much the power transformer will unload, raising B+, with my unknown but lower current load.
(c) You don't know what op point I decided to use, as a best compromise in this amplifier.
(d) The 2A3 quiescent point you have denoted is well above 50 mA. and is of course, totally WRONG.
If you were intelligent, you would NOT have drawn that and then quoted textbook baloney as to distortion and swing, that even a well trained parrot could recite.
Well, maybe you can do better the next time you RUSH to attempt to discredit me C.B. You will have to THINK before you post though.
What does the "P" after your moniker stand for...Industry Professional. OMG. Save us amateur DIYers !!
Until someone offers to make a few measurements on a SV 2A3, I am done with this thread totally - and outta here.
Bye all !!! Have fun.
Jeff Medwin
Walk us through it , please.
Since you were too lazy to read my post, which indicated that the center dot was the quiescent operating point, I've added a blue box to help you.You suggested to Fenderlover an operating point of 37ma/250V. I drew the load line assuming a 2.5K output transformer.
B+ is irrelevant with respect to the operating point. The same operating point could have a B+ of 600V or 275V.
It is easy enough to look at the load line and tell when the tube stops conducting. There's no negative axis for current for a reason...
Edits: 04/01/15 04/01/15
Yes. I plotted it earlier and things got wierd. I dont own a curves stretcher.
Yeah, when you look at what's actually going on in this "de-stressed" 2A3 amplifier, lack of stress isn't actually an inaccurate way of describing things. When you push the output stage, the 2A3 just shuts off for part of the waveform, and gets to take a little break during playback.
Most of us who build single ended amps consider this bad design (or maybe just sloppy design). It isn't that hard to imagine one or two people out there who really like the sound of that much asymmetrical THD.
I believe the whole idea is to give the amplifier a certain 'flavour' . Not only does the inadequately filtered , higher impedence supply give it's own heavy footprint but also the driver . The driver usually being a flimsy device with inadequate current to drive the output stage anywhere near 0 grid volts so the asymmetric clipping not that much of an 'issue' . You get the bandwidth back with the wiring , or so I'm told...
Tons of distortion , rolled off due to the driver and much lower power . May as well use a type 45/4P1L and a higher ratio OPTX IMO for the same output power . If people like the sound of such amplifiers , good on them but I usually prefer in the amplifier as a whole , the output transformer to be the limiting factor
Al
Yeah, I mean, such an amp will sound very different than most others, it's just whether you like or your system needs those limitations imposed on it.
Hiah Al,See this, he is LOCAL to you. Visit him one day.
His first build ever !!
Jeff Medwin
Edits: 04/02/15
Not local at all , it's a nasty 40 mile drive to the other side of town through some of the worst traffic you could possibly imagine . That amp you mention uses a 5k optx strapped on the 8 ohm tap with 16 ohm speakers , if I remember correctly . So his loading is totally different to what you advocate and he also has an active sub for bass .
Al
Al,
40 miles is nothing.
I used to drive a 400 mile truck route every night for years and often in horrible winter weather.
Get up early before the traffic gets too bad.
It is your chance to listen to a JM designed amplifier in a good system.
How can you pass up this opportunity?
Don't you want to hear it for yourself?
DT 667
I'll be perfectly frank with you : I have no time and no interest...
If it's so good , why don't you jump on a plane and hear it yourself ?
Al
RMAF in Denver would be more likely since DF & JM would be there and I could hear the legendary SS product line.
Excuses, excuses, excuses Al.
Jeff
Have you heard this amp ?
Al
No, but I designed it for him. He is more than 40 miles travel distance for me Al.
His FIRST audio build, which I think is unusual and interesting as a DIY start place !!
Nice guy, awfully nice person.
Jeff Medwin
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: