I was unable to get a signal from these when connected to either a mic or line input with 6vdc provided. After trying that I inquired to the sales tech and he said these modules operate in charge mode. That's the part I need help with.
Hi dheming, check out post #67 (page 7), shanx posted a link to an electric violin project that gets into the "charge mode" topic
C2 is absolutely essential, particularly with battery circuits - it 'may' seem to work OK when the battery is brand new, but it will soon begin to work erratically and go unstable as the battery ages even just a little. But you only need one of them, not one for each FET - I would suggest a higher value than 4.7 though, 47 or 100 should be fine.
You do though need capacitors in series with each of the mixing resistors, as you've not got a split supply, and also need a decoupling capacitor on your dummy split supply (from 4.5V to gnd), and remove the spurious R7. Likewise you could remove the spurious R2's which do nothing.
That's not what the internal schematic shows.
I'm not really sure how a charge mode circuit would work from an FET preamp?, I've never seen it done - there's no point, and it probably wouldn't work well?.
But feel free to try it and see.
Are you absolutely sure the devices you have include the FET preamp?.
But even if they didn't, you should still get some results with a normal buffer, you don't need a charge mode amp to make it work, just a high input impedance (100Meg on the internal schematic).
Thanks for the info Nigel. I've updated the circuit with the changes. As far as I understand it, R7 will be the main "gain control". Any other input on the layout and/or values that I have chosen?
The capacitors in the mixer are too low, use 4.7uF or 10uF electrolytics.
C4 needs to be higher as well, again 4.7 or 10uF.
R7 indeed sets the gain (but it was R8 in the previous revision), and you could make it a pot and get rid of the output attenuator. As it's an inverting circuit the gain can be negative as well as positive, and will turn right down - it's a very commonly used configuration commercially.
Would this potentiometer more ideally be linear or logarithmic? Also, for the value, in the last circuit (using the opAmp in non-inverting mode), I had a 220K resistor in the feedback path for the optimum gain. Would a 250K potentiometer be a good starting point, or since the circuit is now quite different would this value be quite different as well?
A 250K pot would give you a maximum gain of 25 times, linear pots usually work pretty well in feedback circuits like this one, you don't normally need a log or anti-log pot.
Just catching up here. Nigel is correct about dheming's charge amp. If the circuit is pre buffered with a fet , a charge amp not required. The output would be a voltage mode at that point and should work with voltage type input stages.
The charge amps are multiplying charge and then convert to voltage. They are quite useful in longer cable runs from source to preamp, and dealing with high frequency cable losses. It is not absolutely necessary in a built in guitar preamp to use charge amps, just an alternate way.
Sounds like Lid55 is putting final touches on the preamp..I hope a sound sample is coming soon (!)
The charge amps are multiplying charge and then convert to voltage. They are quite useful in longer cable runs from source to preamp, and dealing with high frequency cable losses. It is not absolutely necessary in a built in guitar preamp to use charge amps, just an alternate way.
Sounds like Lid55 is putting final touches on the preamp..I hope a sound sample is coming soon (!)
Hi Nigel, thanks for the info... I tried the pot in the loop, but thus far... it's very unstable (audio cuts out at certain points, then jumps into distortion, then silence again further on). Could the voltage divider resister values have anything to do with it?... additionally, I did a search on this forum and came across this thread:
http://www.diyaudio.com/forums/solid-state/37542-volume-controll-feedback-loop-2.html
seems like it's quite a complicated way to go at the very least. Have you had success with gain controls in the feedback loop? Do you have to wire the pot a special way?
It sounds like your pot is duff?, a noisy pot would be really bad news in a feedback circuit - if the slider goes O/C the gain will go to absolute maximum (VERY, VERY high). It's worth wiring the slider to one of the ends, then if the slider goes O/C the gain just goes to the maximum value of the pot.
Well... I've now been trying different value resistors without a pot, just to simplify things.. and I find that... the opamp might be unstable with the current component values. I say "might" because I can't rule out that my sound card is doing something unpredictable with the high impedance/low impedance internal switching... I'll do some tests with the other soundcard
But so far... with a 100K resistor in the feedback circuit... things go well, the audio quality is right and the gain is right... but then... no audio... sometimes it cuts back in for a bit then goes out again. If I disconnect the power for a moment then reconnect, then the audio is alright again for a little, till it starts cutting out again. If I try a 1M resistor, the circuit doesn't work at all. With a 10K resistor, the circuit seems stable, but there is not enough gain. Any ideas?
But so far... with a 100K resistor in the feedback circuit... things go well, the audio quality is right and the gain is right... but then... no audio... sometimes it cuts back in for a bit then goes out again. If I disconnect the power for a moment then reconnect, then the audio is alright again for a little, till it starts cutting out again. If I try a 1M resistor, the circuit doesn't work at all. With a 10K resistor, the circuit seems stable, but there is not enough gain. Any ideas?
Well 1M is trying to give you a gain of 100 times, and 10K only gives you a gain of 1 (i.e. no gain at all). If 100K gives the right gain, then stick 100K in.
How are you constructing it?.
Hi Nigel, I've attached the circuit I'm testing.
Well honestly I think the ideal gain I want is around 220K again. The point is though, even 100K only works for a while, then the audio cuts out... it's like something in the circuit is slowly building up power... then after a certain threshold it shuts off. I immediately think capacitor... but... I've tried other values for the capacitors and haven't figured it out. All I can think right now is that there needs to be some very specific combination of resisters and capacitor values needed... and I'm looking for some formula to figure it out. The formula Shanx posted a couple pages back on post96:
v(out) = -Rfeedback x (v1/Rwiper1+v2/Rwiper2+v3/Rwiper3)
Not sure if this is my issue or not... it's beyond my comprehension of electronics at the moment. I'm not sure what voltage out I want for this circuit.... I know it's 4.5V in (into the opamp)... therefore, is the same voltage desirable on the output for stability?
Well honestly I think the ideal gain I want is around 220K again. The point is though, even 100K only works for a while, then the audio cuts out... it's like something in the circuit is slowly building up power... then after a certain threshold it shuts off. I immediately think capacitor... but... I've tried other values for the capacitors and haven't figured it out. All I can think right now is that there needs to be some very specific combination of resisters and capacitor values needed... and I'm looking for some formula to figure it out. The formula Shanx posted a couple pages back on post96:
v(out) = -Rfeedback x (v1/Rwiper1+v2/Rwiper2+v3/Rwiper3)
Not sure if this is my issue or not... it's beyond my comprehension of electronics at the moment. I'm not sure what voltage out I want for this circuit.... I know it's 4.5V in (into the opamp)... therefore, is the same voltage desirable on the output for stability?
me too! what I'm thinking of recording:
- the three individual piezos straight to soundcard (so... 3 different recordings)
- the three straight piezos combined (simply wired in parallel)
- the three individual piezos with the buffer circuit (3 different recordings)
- the three individually buffered piezos combined
- the three buffered and amplified piezos combined
the reason I want to test and wire in a gain stage is because my portable soundcard can't add enough gain... my other soundcard has enough gain... but I can't use it with my portable rig
It's no good randomly changing capacitor values, which would cause the effect anyway - capacitor values are rarely critical, and NEVER for this reason.
C3 etc. are the wrong way round though, that might be causing your problems?.
C2 etc. are still too low, and will lose bass - should be electrolytic with the positives to the FET's.
Not at all, opamps are VERY, VERY easy to design with, that's the entire point of them.
That seems exceedingly overly complicated and confusing?
The gain of the stage is simply the feedback resistor (220K) divided by the input resistor (10K) so 22 times. You don't need to consider the 'adding' at all, particularly as each input is adjustable.
That's the DC level on the output, and isn't relevant to anything.
Thanks again for the info Nigel. Guess what... the circuit works correctly now. not sure if the reversed capacitors were the issue... as it still works with them reversed in the wrong direction, but it's hard to tell... they might have had an effect. I think what was needed was to power off the sound card... reset it from all the torturous on-the-fly capacitor changes.
1uF film caps should work well here too, no?
1uF film caps should work well here too, no?
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