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High gain noise management
High gain noise management
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Old 8th April 2019, 05:32 PM   #1
Max999 is offline Max999
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Default High gain noise management

Hi all,

I have built an amp that basically runs 3 12ax7 halves flat out into eachother. It has a good amount of gain but it is noisy ( hummy and ofc hissy)

What I am trying to understand is this: what if I add resisitive dividers per stage but add another one .. would this be quieter?
You throw away noise with a divider but you also throw away signal, so signal to noise ratio stays the same, correct? So looking at it like this nothing would change. And the extra stage would add hiss/hum also probably.

Most high gain amps I see are based on Soldano's work and they use a lot of dividers and are quiet but maybe the dividers are more for reducing blocking distortion.
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Old 8th April 2019, 05:41 PM   #2
dreamth is offline dreamth  Romania
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If you never built something like this , you don't know what SNR means:
CARVIN QUAD X-AMPLIFIER SCH Service Manual download, schematics, eeprom, repair info for electronics experts
This thing is totally quiet at max gain.
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Old 8th April 2019, 07:17 PM   #3
jan.didden is offline jan.didden  Europe
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The noise performance is dictated by the first stage. The usual routine for lowest noise from multiple stages is to give the first stage the highest gain possible. That will give it the lowest input-referred noise.
Try something like current source loading.
And then see if you can cut back on the number of stages. Do you really need three?

Jan
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Old 8th April 2019, 07:25 PM   #4
DPH is offline DPH  United States
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Second question is what's the closed loop gain vs the amount needed to clip/overdrive (and exactly how you want that to behave). A modest amount of feedback, either local or global, might pull this back from the brink.
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Old 9th April 2019, 01:50 AM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by jan.didden View Post
... cut back on the number of stages. Do you really need three?
Quote:
Originally Posted by DPH View Post
...A modest amount of feedback, either local or global, might pull this back from the brink.
Gentlebeings-- This is a guitar amp. MORE MORE MORE gain!! No NFB, we want the sweet(?) sound of 12AX7 abuse!!

Attaching the more obscene part of Carvin's vile plan.

The eleven cascaded stages, with interstage loss, might be a good FM limiter. I'm starting a sim of this plan but both my fingers and my sim are tired tonight.

edit-- sim chokes on 8 stages (eval version). Here's 4 stages, drive levels 10mV to 1V peak.
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File Type: gif Carvin-Quad-X-Amp-chain.gif (36.8 KB, 77 views)
File Type: gif Carvin-Quad-X-Amp-4-chain.gif (41.1 KB, 76 views)
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Old 9th April 2019, 01:55 AM   #6
thoglette is offline thoglette  Australia
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Once you've solved all your other problems, stop expecting low-noise performance from common-or-garden variety tubes. The 7025 is the first stop as it's basically a better 12AX7

And while I think of it,
a) bandwidth limit your first (noisy) stage. Hard.
b) clean all the crap out of the ground and power for that stage.

Those are the easy things. The hard things are all the stuff we know about signal routing and cross talk.

Last edited by thoglette; 9th April 2019 at 02:03 AM.
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Old 9th April 2019, 02:38 AM   #7
thoglette is offline thoglette  Australia
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Oh, and lose the carbon composition resistors.
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Old 9th April 2019, 03:22 AM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Max999 View Post
Hi all,

I have built an amp that basically runs 3 12ax7 halves flat out into eachother. It has a good amount of gain but it is noisy ( hummy and ofc hissy)
3 12ax7 halves give 100*100*100 gain which is 1,000,000 !
So not surprising you are getting massive noise.

Screen input signal. Ground plane on your pcb.
Keep transformers and high volts ac away from audio signal etc etc
Keep heater wires away from audio.
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Old 9th April 2019, 05:46 AM   #9
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by PRR View Post
The eleven cascaded stages, with interstage loss, might be a good FM limiter. I'm starting a sim of this plan but both my fingers and my sim are tired tonight.
Fascinating.

I'm contemplating how to get a "hyper-clean" sound and this is fascinating. Particularly the sets of pairs (each pair having a "normal" stage; an attenuator; then a cold clipper) and the absence of cathode bypass caps. I wonder if the bias shift (in your simulation) has an audible effect?

Last edited by thoglette; 9th April 2019 at 05:49 AM.
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Old 9th April 2019, 06:08 AM   #10
dreamth is offline dreamth  Romania
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Now i remember that I built(11 years ago) only the 5 stages circuit and compared with many 2 stages high gain circuits, like Mesa(soldano-slo100), it had no noise but a considerable level of drive...I never had the balls to clone the 11 stages...hopefully 2 X "nothing"= " nothing" +3db...everybody liked more the Mesa(soldano) sound for drive though...the main difference was in the voltage supply, Carvin had 3 times lower supply than Mesa...so it worked a bit as a starved anode too...

Last edited by dreamth; 9th April 2019 at 06:14 AM.
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