I've marked up the schematic to show how the volume control should be positioned at the input of the PA. Stated in words, you're modifying where one end of C1, C6, and C18 are placed in the circuit. C6 should be rated at 35V or more; this will ensure that C6 can handle failure of IC2, should that ever occur.
As mentioned earlier, you can improvise an added pot at the PA input (either log or linear taper should be OK, since it's only experimental). But depending upon how the caps are installed, it may be easier to rewire the cap connections with the existing volume control, since you can always reverse the changes if necessary.
Since this modification only changes the volume control placement within the amp without changing any circuit gains, you may be able to get an approximate feel for the expected hiss improvement before you begin any changes: adjust volume to a typical level that you use, remove program signal and note hiss level. Then restore program signal and get a sense of how many dB additional volume is available at full volume--- say it's 10dB for example. Assuming my thinking isn't cockeyed, I think you'll get about 10dB reduction in hiss when you alter the volume control location. You might also consider revising R28 to 0 ohms. Given a 10k volume pot, shorting R28 should improve S/N an additional 2dB. All this assumes that the existing configuration show no change in hiss as you vary volume from min to max.
Let us know what you experience. Good luck!
As mentioned earlier, you can improvise an added pot at the PA input (either log or linear taper should be OK, since it's only experimental). But depending upon how the caps are installed, it may be easier to rewire the cap connections with the existing volume control, since you can always reverse the changes if necessary.
Since this modification only changes the volume control placement within the amp without changing any circuit gains, you may be able to get an approximate feel for the expected hiss improvement before you begin any changes: adjust volume to a typical level that you use, remove program signal and note hiss level. Then restore program signal and get a sense of how many dB additional volume is available at full volume--- say it's 10dB for example. Assuming my thinking isn't cockeyed, I think you'll get about 10dB reduction in hiss when you alter the volume control location. You might also consider revising R28 to 0 ohms. Given a 10k volume pot, shorting R28 should improve S/N an additional 2dB. All this assumes that the existing configuration show no change in hiss as you vary volume from min to max.
Let us know what you experience. Good luck!
I will definitely try that as soon as can make the time - thank you! It may be a little while, but I'll report back.
Before I launch into this further I have a question regarding some component values which are different on my amp to those on the diagram - how would these differences affect things, noise/hiss in particular?
The differences are:
My R25 is 270R not 680R (120R)
My R26 pot is 1K not 2K2 (470R)
My C16 is 47uF not 22uF (47uF)
My R6 is 390R not 100R (390R)
My R9 is 180R not 470R (180R)
The note on the diagram states: 'Values in brackets used for higher gain in tone control and complementary lower gain in Mag PU amp', so some of these differences are explained as alternatives, but not R25 or R26
The differences are:
My R25 is 270R not 680R (120R)
My R26 pot is 1K not 2K2 (470R)
My C16 is 47uF not 22uF (47uF)
My R6 is 390R not 100R (390R)
My R9 is 180R not 470R (180R)
The note on the diagram states: 'Values in brackets used for higher gain in tone control and complementary lower gain in Mag PU amp', so some of these differences are explained as alternatives, but not R25 or R26
I'm sorry for my slow response. I've been caught up in our elections. 😀
Re R9: I want to confirm we're looking at the same resistor. I see R9 as providing bias current to the TR2, TR4 current source within the PA, and the schematic indicates the values in K ohms rather the R ohms. Same focus? Not sure what might have motivated changes in this resistor, but 180k looks reasonable.
C26 variations are to retain similar corner frequencies in concert with associated variations in R25, R26 values.
The voltage gain of the PA stage is 1+R11/R6; R11=22k, R6=1k in the OP schematic. Corresponding PA gains are:
R6= 1k; G= 23 = 27.2dB
R6= 390R; G= 57.4 = 35.2dB
R6= 100R, G=221 = 46.9dB
The values of R25 and R26 present in your version result in larger adjustment range in left/right balance, and correspondingly greater hiss amplitudes.
Re R9: I want to confirm we're looking at the same resistor. I see R9 as providing bias current to the TR2, TR4 current source within the PA, and the schematic indicates the values in K ohms rather the R ohms. Same focus? Not sure what might have motivated changes in this resistor, but 180k looks reasonable.
C26 variations are to retain similar corner frequencies in concert with associated variations in R25, R26 values.
The voltage gain of the PA stage is 1+R11/R6; R11=22k, R6=1k in the OP schematic. Corresponding PA gains are:
R6= 1k; G= 23 = 27.2dB
R6= 390R; G= 57.4 = 35.2dB
R6= 100R, G=221 = 46.9dB
The values of R25 and R26 present in your version result in larger adjustment range in left/right balance, and correspondingly greater hiss amplitudes.
Sorry to hear you were caught up in the election - at least you're smiling!
The R6 and R9 are both on the Mag PU board - I should have said - the other resistors and capacitors are on the pre amp board with the balance pot. Maybe I could swap the values for the lower gain tone control and higher gain PU option? And lose the balance control altogether? I don't really need it, but didn't much like the loss of level despite the lack of hiss.
The R6 and R9 are both on the Mag PU board - I should have said - the other resistors and capacitors are on the pre amp board with the balance pot. Maybe I could swap the values for the lower gain tone control and higher gain PU option? And lose the balance control altogether? I don't really need it, but didn't much like the loss of level despite the lack of hiss.
Thanks for directing me to the correct module Ref Des. Makes way more sense now.
I agree that it might be possible to modify values in the Mag PU board, but I don't see how that would help the situation when using line input sources. Am I missing something?
I can't tell with any confidence how the volume control is physically connected to the circuit board. Are there interconnecting wires from the controls to the PCB, or do the controls have leads that solder directly into the circuit board?
A similar question re the balance control--- is it in fact a single element pot and how is it connected?
I still believe moving the volume control to the PA input would give the most dramatic improvement, but perhaps changing resistor values might present an acceptable compromise between hiss amplitude and adequate gain. That would certainly be easier and more esthetic. I suggest changing R24 (and its brother in the other channel) to a smaller value to lower decrease hiss, but hopefully having sufficient gain for listening at elevated SPL.
Any comments/guidance?
I agree that it might be possible to modify values in the Mag PU board, but I don't see how that would help the situation when using line input sources. Am I missing something?
I can't tell with any confidence how the volume control is physically connected to the circuit board. Are there interconnecting wires from the controls to the PCB, or do the controls have leads that solder directly into the circuit board?
A similar question re the balance control--- is it in fact a single element pot and how is it connected?
I still believe moving the volume control to the PA input would give the most dramatic improvement, but perhaps changing resistor values might present an acceptable compromise between hiss amplitude and adequate gain. That would certainly be easier and more esthetic. I suggest changing R24 (and its brother in the other channel) to a smaller value to lower decrease hiss, but hopefully having sufficient gain for listening at elevated SPL.
Any comments/guidance?
I'm away from home for a couple of days, but I'll take some photos as soon as I can. The latest development is that the speaker relay seems not be be working (but speakers stay connected) and there is a rustling sound on one channel ( the last time I heard a very similar sound on another amp, it was a faulty transistor. Who knows this time, but I'll see if I can remedy it anyway)
Many thanks for the continued support!
Many thanks for the continued support!