| Question and opinion [message #11244] |
Tue, 23 June 2009 00:03  |
BC
Messages: 583
Registered: March 2000
Location: Kentucky
|
Senior Member |
|
|
I was talking to a knowledgeable amp tech yesterday about Kustom amps and their noise issues. During the discussion the question of gain control came up. Understand, most of my experience with Kustom has been the K200B series which is hands down my favorite. He said there was a way to reduce the output of the power amp section by making an adjustment. This would in turn reduce the "quick gain" of the volume control and give it more useable range without cutting the final output. This idea interests me. Of course my technical knowledge goes about as far as off, on and volume. How about it? BC 
|
|
|
|
|
|
|
|
|
|
| Re: Question and opinion [message #11255 is a reply to message #11247] |
Tue, 23 June 2009 23:34  |
BC
Messages: 583
Registered: March 2000
Location: Kentucky
|
Senior Member |
|
|
|
Steve....I was told that everything behind the first stage is pretty well wide open in power. Since the noise is usually in the front of the amp and stepped up all along the way it keeps increasing as well. I am a player and not a technical guy so I was seeking second and third opinions. Thanks for the reply! BC
|
|
|
|
|
|
| Re: Question and opinion [message #11260 is a reply to message #11257] |
Wed, 24 June 2009 09:26 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
|
Senior Member |
|
|
The Kustom preamp stages are running wide open and the volume is controlled through negative feedback. That is, some of the voltage is sent back from the output of the preamp back to the input of the preamp. This is what set Kustom apart from more conventional audio stages. The only trouble is that without a feedback signal, the output is wide open and at maximum. This is why the amp will produce volume at 0, fully counterclockwise. I suspect what the mod is, a resistor is added to the blue lead or the mixer resistors on the power amp board are increased to allow a higher voltage from the preamp for a given power output to the speakers. This would extend the useful range of the preamp. We all typically see full wttage at the 10:00 position. There might be some usefullness in that. Last night, I had my K100 just cracked above 0 and I had more than enough sound, but the sound was very thin. This might be something to consider.
Conrad
|
|
|
|
| Re: Question and opinion [message #11261 is a reply to message #11244] |
Wed, 24 June 2009 11:37 |
chicagobill
Messages: 2005
Registered: April 2003
|
Senior Member |
|
|
Conrad:
What version amp are you referring to? My recollection is that the B-series pre-amp's volume control while being part of a "feedback" loop, did two things: 1-grounded out the signal (CCW) and 2-increased the signal gain of the first transistor stage(CW).
The Frank heads have their volume control at the input to the pre-amp, so they always make a certain amount of noise.
I've found that replacing some of the carbon comp resistors in the power amp with carbon film or metal film resistors, will make a huge difference in background noise and hiss.
BC:
To me reducing the power of the output stage and increasing the volume of the pre-amp will lessen some of the power amp noise, but it will increase the apparent pre-amp noise. It's treating the symptom, not curing the disease.
|
|
|
|
| Re: Question and opinion [message #11267 is a reply to message #11261] |
Thu, 25 June 2009 08:54 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
|
Senior Member |
|
|
I was kind of working off of memory. I had schematics for K100 and K200B. I can't find them now. I thought the volume was controlled by negative feedback in the first stage of the preamp rather than the typical voltage divider pots that every other amp uses. By feeding part of the output signal back to the input of the preamp, the 2 siganls subtract from each other leaving the difference to be passed on. Ideally, the volume can never be totally 0. Some signal must pass through. That seems like Kustom. I know none of my amps go to 0. Even at full CCW, the amp can have quite a bit of output but then drop as you raise the volume. Then it increases rapidly up until about 10:00. After that you are just driven out of the room. It would ssem that reducing the voltage to the power stage would allow more control of the preamp. Kind of like those Marshalls on Spinal Tap. These things go to 11.
Conrad
|
|
|
|
VintageKustom.com
Search
Help
Members
Register
Login
Home
