About preamp gain

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Leevi
Junior tube assistant
Posts: 106
(2/21/04 2:31 am)
 About preamp gain
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 Hi,

I'm wondering the different ways to build preamp especially
what comes to their amplification.

E.g. in the VOX top boost channels there are 1/2 x 12AX7 1 x 12AX7 PI. The second 12AX7 was originally added to the top boost channel in order to ensure the power for the tone stack, right ?

If you look at Matchless (e.g. DC30 or Lightning) the first amplification stage (12AX7) is connected parallel which makes gain factor 50 50 as someone wrote under other topic. I would say the gain increases too much in this kind of preamp. The same has probably Matchless noticed as well because they have added a voltage divider after the
tone stack in order to drop the gain.

Why to gain the signal if you have to decrease it with resistors? Someone wrote in some article that the amplification has to be the maximum in the first amplification stage in order to get the best possible signal noise-relation. Maybe that is true, but according to my experience you'll get other problems like hum
and other disturbances (microphone phenom for instance). I think the best sound can be reached with a simple circuit where the number of components is low.

/Leevi
 
Tiny Daddy
I will work on all amps
Posts: 508
(2/21/04 9:36 am)
 Re: About preamp gain
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 The first stage needs to be low-noise because any noise in this stage is amplified by following stages. For best results the first stage needs to have SOME gain but not maximum gain.
The reason for all the extra amplifier gain: Electric guitar relys on the amplifier stages to overload and produce that electric guitar sound. But that good sound cannot be accomplished in one gain stage; it sounds much better if most or all of the stages overload. Sometimes in sequence, sometimes all together, depends on the design. And overloaded tubes sound a whole lot better than overloaded transistors or op-amps.
 
Leevi
Junior tube assistant
Posts: 107
(2/21/04 12:15 pm)
 Re: About preamp gain
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 OK, overdrive is a special case where you need gain in order to get that sound. I was thinking more about a clean guitar sound. I think the first amps aimed just at clean sound. The tubes are active components and they certainly richen the sound as you described. The passive components such as resistors and caps that are placed on the signal path take something away from the signal, a potentiometer is probably the worst in that sense. I fully agree to your comment on tube amp overdrive vs. transistor amp overdrive.
/Leevi


 
coap01
Junior tube assistant
Posts: 104
(2/21/04 5:11 pm)
 gain
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 the problem with gain is that its difficult to control. you asked why do gain the signal and then use resistors, the answer for that is otherwise, it simply sounds like crap (in overdrive case). i did an experiment in the amp i'm currently working on. i took out those resistors that attenuate the signal, and the result was horrible sound. the amp didnt break into oscillation but the distortion sound quality was awful.

i think the idea behind gain is to have one stage add gain to the signal to the point of clipping (or compression) or whatever you wanna call it, then take that clipped signal and reduce it so that the next gain stage can do the exact same thing. Obviously we can control exactly how much gain is added by each stage. Note that this is for the overdrive/distortion case.

For clean case, i'd imagine you'd wanna make sure that you dont add too much gain and cause the signal to be clipped
 
geoffreyreed
I will work on all amps
Posts: 315
(2/21/04 11:29 pm)
 Re: gain
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 COap, you got that right distortion sounds better, richer than hard clipping. Transistor amps typically hard clip. Snip snip to the top of the waveform. all the harmoinic richness is lost. multiple layers of a few percent distortion is much more interesting sounding, richer. geoff
 
Leevi
Junior tube assistant
Posts: 108
(2/22/04 11:23 am)
 gain
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 Thanks for the comments.
There is probably not an optimal way to build a preamp. It depends on too many things and listeners. I'll start soon an AC30 head project. Although I have built several AC30 based amps before I'm sure that I'm going to battle again with the gain/hum/noise problems. Maybe that belongs to the hobby, anyway I like it.
/Leevi
 
 
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