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In Reply to: RE: SUN SV 2A3 stereo SE amp .... MEASUREMENTS posted by rage on April 17, 2015 at 13:28:27
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
Follow Ups:
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?
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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.
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Big speakers and little amps blew my mind!
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