Is there a way to make this circuit gain adjustable? I have been using this preamp into a class D power amp with volume control. Recently I purchased two mono block tube power amps which are 14dB gain non adjustable. I need to add in an active gain control somewhere between my turntable and speakers. Here is the schematic of the preamp, it is the boozhound labs preamp kit.
(http://diyaudioprojects.com/Solid/JFET-Phono-Preamplifier-Kit/Boozhound-Labs-JFET-Phono-Preamp-Kit-Schematic.png)
I'm sure making either the first or last stage adjustable will result in some compromise with input or output impedance. I'm not too familiar with JFET's to fully understand this. I'm also open to the idea of building a dedicated stereo voltage gain circuit if that would be more practical.
Why active? This is a great place for a 10K passive stepped attenuator (which in and of itself is a cool project, or can be sourced cheaply on ebay). You can't (shouldn't) add adjustable gain to the RIAA preamp because the gain is not applied evenly to the entire frequency spectrum (corrects for record cutting head de-emphasis in order to maximise headroom on a well loved but flawed medium) and has to be structured the way it is for that to cancel out (kinda like Dolby NR with the high end compression and then expansion on playback, but with EQ - the additive must be equal and oppisitte to the subtractive portions of the 'system') - it would have to be after the RIAA preamp and seperate. Too much gain interstage and you'll make the monoblocks unhappy. If you need more volume you should get bigger amps.
Makes sense, but would I want the stepped attenuator between the preamp and monoblocks?
Yes, exactly.
Thank you! One last question, it seems 10k and 100k are most common values for stepped attenuator kits on ebay. which would be better? On one hand it seems 10k would be better since it offers less series resistance (overall across all stepped settings) between preamp and power amp. On the other hand the 100k offers less loading of the preamp due to greater resistance to ground (across all settings).
What do you want? More gain or less gain?
14dB gain in a power-amp is mighty slim. Also a classic phono system has another 20dB gain Line Amp. Which is why I'm thinking you may want "more".
If you just want "less", the 100K resistor R10 begs to be replaced with a 100K potentiometer, Q2 Gate to wiper. Any error from pot being 90K or 110K is below 50Hz, where room and speaker responses cause far worse errors.
Quote from: PRR on October 20, 2015, 03:33:57 PM
What do you want? More gain or less gain?
I simply want a "volume" control. Whether that means an active stage, or just a passive attenuator... I know for now that I need a means to adjust the volume of my records I'm listening to because my power amp no longer has a built in volume control. According the preamp kit it should have about 40dB of gain already (30dB for each JFET stage minus ~20dB for the passive EQ stage). Something tells me the ~40dB preamp gain and ~14dB power amp gain will be enough so perhaps I can try the passive attenuator as suggested by Granny.
Try a passive pot. It's cheap and functional. If that preamp has sufficient supply voltage, it won't overload, so the pot can go between preamp and power amp.
However, 40dB+14dB is 54dB or gain of 500. Taking phono needle level as 5mV, you have 2.5V at speaker which is about 1 Watt. I suspect it won't go "loud".
What I used to do was rig an opamp for gain of 5 or 10, a Line Amp. Preamp, pot, line amp, power amp. That's a fairly traditional hi-fi system gain scheme.
Should be loud enough with some efficient loudspeakers. Anything modern and commercial will be rather quiet. ... but wait, what sort of monoblocks put out <1W? Can that be right?
Quote from: Granny Gremlin on October 21, 2015, 08:46:32 AM
... but wait, what sort of monoblocks put out <1W? Can that be right?
The sort that are being fed with a tiny signal. Like Paul said you don't generally feed a phono amp straight into a power amp, you have a preamp in between to bring the levels up some more.