So I've decided to build this preamp to merge with a power amp build I finished. This is not my design and thought I'd see what suggestions are offered and in particular if there are any glaring problems with this schematic? This is a high gain style circuit that I believe is based on the JCM 800 but sounds much better.
Does it look right or should I change component values somewhere?
Regards,
Ccat.
Does it look right or should I change component values somewhere?
Regards,
Ccat.
Attachments
Three things struck me as being questionable:
1. The value of R18 and R20 seem a bit high at 1Meg. Most channel mixing resistors are around 220K or so. Will this affect the signal too much?
2. On the clean channel, having the gain make up stage after the tone stack be a cathode follower. Since the gain of a cathode follower is always less then unity, will there be enough signal strength to compensate for the insertion loss from the tone stack? A better place for this is like in the lead stage driving the tone stack.
3. Not such a biggie, but no value is given for the grid stopper R2 at the input. Maybe around 10-20K? I think many amps have this resistor value too high. This resistor can be a source of noise. Since it is in the input, this noise is amplified the most because it is amplified by the most gain stages. So many amps merely copy Fender's value of 68K(34K for the hot input if configured that way).
1. The value of R18 and R20 seem a bit high at 1Meg. Most channel mixing resistors are around 220K or so. Will this affect the signal too much?
2. On the clean channel, having the gain make up stage after the tone stack be a cathode follower. Since the gain of a cathode follower is always less then unity, will there be enough signal strength to compensate for the insertion loss from the tone stack? A better place for this is like in the lead stage driving the tone stack.
3. Not such a biggie, but no value is given for the grid stopper R2 at the input. Maybe around 10-20K? I think many amps have this resistor value too high. This resistor can be a source of noise. Since it is in the input, this noise is amplified the most because it is amplified by the most gain stages. So many amps merely copy Fender's value of 68K(34K for the hot input if configured that way).
One other quick thought- if the clean channel doesn't have enough gain, increase R10 to 1Meg. This will give less signal loss at that point. I see that you already have a question mark by it on your schemo. Unless it needs to stay at that value for the optocoupler switching design. It looks to be a resistance separating the two legs of that device maybe?
Are the optos for channel switching or just gain adjustment? If they're for channel switching I would suggest putting them from the wiper of each treble pot to ground, as this should provide better offness.
You'll also want a pair of back-to-back 15V zener diodes from the output (after C15) to ground. You don't want to be shoving hundred-volt signals into effects pedals!
You'll also want a pair of back-to-back 15V zener diodes from the output (after C15) to ground. You don't want to be shoving hundred-volt signals into effects pedals!
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