The importance of guitar amp input circuits

SoulFetish · Post by **SoulFetish** » Sat Jan 31, 2015 1:10 pm

I'd like to start a discussion dedicated to exploring different techniques in designing input circuits for guitar amps.
Perhaps a tutorial covering the importance of radio frequency suppression, signal to noise, microphonics, shielding, miller capacitance(and how it relates to series resistance, cable/shielding capacitance), etc.
Different amp designers sometimes use different methods of designing input circuits. Some might find it helpful to see an explanation as to how these circuits work, some equivalent circuits, and how to make calculations for their own designs.
Any interest?

pops · Post by **pops** » Sat Jan 31, 2015 2:50 pm

I think it would be and interesting discussion. I would like to know how to change the input impedance of a tube amp to match with a piezo to not use a preamp. It seem to work fairly well, but knowing more about changing would be interesting and useful. Thanks

roberto · Post by **roberto** » Sat Jan 31, 2015 3:16 pm

Hi, this forum already contains all those infos.
Use the search function.

tsutt · Post by **tsutt** » Sat Jan 31, 2015 3:51 pm

I for one for one find the search function here absolutely worthless. i have never found anything here with it. i have had to search all the threads one by one.

M Fowler · Post by **M Fowler** » Sat Jan 31, 2015 4:05 pm

pops wrote:I think it would be and interesting discussion. I would like to know how to change the input impedance of a tube amp to match with a piezo to not use a preamp. It seem to work fairly well, but knowing more about changing would be interesting and useful. Thanks

Try a 10m pot on input jack to vary from 150k for humbuckers to 10m for pedal steel guitars and piezo.

roberto · Post by **roberto** » Sat Jan 31, 2015 4:14 pm

tsutt wrote:I for one for one find the search function here absolutely worthless. i have never found anything here with it. i have had to search all the threads one by one.

I don't agree. It's like saying the cars are useless just because I don't have the driver license.
You have to learn how to search. I've just tried "how to piezo" restricted in the technical discussions, and I've found all the info.

SoulFetish · Post by **SoulFetish** » Sat Jan 31, 2015 4:32 pm

Roberto, I did use the search function. Im not sure reading through 15 pages listing non specific threads to see if 20 of them contain bits of info is very efficient or practical.

Pops, that wasn't the direction i was thinking of going with this when i first posted. But that is a great question and there are some interesting answers. Lets expand this to deal with exploring the entire first stage. To your point, i learned a few circuits with higher input impedences I learned reading Blencowe. I'll share and explain those with his permission. Peizio pickups are the worst when heavily loaded, it sound someone throwing rocks at a banjo.

ToneMerc · Post by **ToneMerc** » Sat Jan 31, 2015 4:35 pm

SoulFetish wrote:I'd like to start a discussion dedicated to exploring different techniques in designing input circuits for guitar amps.
Perhaps a tutorial covering the importance of radio frequency suppression, signal to noise, microphonics, shielding, miller capacitance(and how it relates to series resistance, cable/shielding capacitance), etc.
Different amp designers sometimes use different methods of designing input circuits. Some might find it helpful to see an explanation as to how these circuits work, some equivalent circuits, and how to make calculations for their own designs.
Any interest?

http://www.aikenamps.com/index.php/white-papers

TM

SoulFetish · Post by **SoulFetish** » Sat Jan 31, 2015 5:11 pm

Aiken's tech pages are must-have bookmark for anyone interested in tube amps. I recommend reading everything there, then reading them again several more times.
His paper on Miller capacitance is particularly helpful In this area (as well as the paper on gridstoppers and resistor noise). Anyone curious about why the resistor value of 68k is seen at the input of many classic amps should start here.
Personally, i love old classic amps but think 68k is way too large a resistor value. There are many other ways of achieving a -3db cutoff frequency of around 20kHz without large series resistances.

Jana · Post by **Jana** » Sat Jan 31, 2015 5:12 pm

This would also be a good place to start:

http://ampgarage.com/forum/viewtopic.php?t=9023

All of the criteria the OP listed are a part of the basic design of a tube stage—learn those things and then you know how to tweak it or "design" for your specific needs and goals.

johnnyreece · Post by **johnnyreece** » Sat Jan 31, 2015 5:16 pm

I rarely use sites' built in search functions anymore. I prefer to go to Google and search using the terms I'm looking for, followed by site:ampgarage.com.

Edit: Sorry for the derail...

SoulFetish · Post by **SoulFetish** » Sat Jan 31, 2015 5:30 pm

Im bailing on this thread i think. Its going off the rails. There's a reason i didn't just post a link to download the 4th edition of Radiotron Designers Handbrook and say "If you wanna know anything about the input of Guitah amps, dude, read this. It's in there"

ToneMerc · Post by **ToneMerc** » Sat Jan 31, 2015 5:42 pm

SoulFetish wrote: Personally, i love old classic amps but think 68k is way too large a resistor value. There are many other ways of achieving a -3db cutoff frequency of around 20kHz without large series resistances.

You can hear your guitar at 20kHz?

TM

Leo_Gnardo · Post by **Leo_Gnardo** » Sat Jan 31, 2015 6:08 pm

SoulFetish wrote:Im bailing on this thread i think. Its going off the rails. There's a reason i didn't just post a link to download the 4th edition of Radiotron Designers Handbrook and say "If you wanna know anything about the input of Guitah amps, dude, read this. It's in there"

Before you go off the rails, try 1: Google search, don't use the site's search, some find Google works better. Type Amp Garage into your search if you want answers specfically on this site. and B: there's a terrific sticky right at the top of this stack "preamp gain stages, how they sound and why." Lots of good answers right there: no need to reinvent the wheel. Read. Research. Solder & listen. If there's something you still don't get after that, just ask.

martin manning · Post by **martin manning** » Sat Jan 31, 2015 6:20 pm

SoulFetish wrote:Im bailing on this thread i think. Its going off the rails. There's a reason i didn't just post a link to download the 4th edition of Radiotron Designers Handbrook and say "If you wanna know anything about the input of Guitah amps, dude, read this. It's in there"

People have pointed to information which describes the theory and practice of common cathode gain stages as used in guitar amps. This is very specific information that addresses the original question, not the whole of RDH4. I'll add that Merlin Blencowe's web pages are another good source. Chapter one of his book is download-able free, and it covers all of this in much better style than anyone here is willing or even capable of reiterating.

The Amp Garage

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