DIY Diamond Buffer Preamp

[Firebottle]

The difference 6.8 to 7pF would be nothing.

But 7pF is an odd value as it doesn't sit in the E12 range of values, eg 4.7, 5.6, 6.8, 8.2, so are you sure the cap is 7pF?

The circuit shouldn't give bass emphasis or treble cut as stands, as Andrew says.

[Puffin]

All I have to go on is they are marked "7" and RoHS. My gadget to check capacitors needs a 9v battery. Will get one when I go to the doctors in 10 mins!

[RothwellAudio]

"7" is probably some kind of manufacturer's batch number or a code for something other than the capacitance.
Can you post the circuit diagram?

[Puffin]

Here it is. Not a very good copy I am afraid, but that is how it came to me.



My PDF version is readable if enlarged. I could add it as a PM if neccessary.

[RothwellAudio]

Sorry, I can't read that properly.

[Puffin]

I could try to sketch it out and photograph it. Is there any particular section or do you need the whole thing?

[RothwellAudio]

The power supply/regulator bit isn't necessary, and we only need to see one channel, so the bit round one op-amp and its associated output transistors is the important bit.

[Puffin]

Hope this works!

[RothwellAudio]

R1 and C1 (1k and 220pf?) form an RF filter. If you have the value of one of them way off you could roll off the treble in the audible band.
R3 sets the gain in conjunction with R5. C2 is meant to compensate for the extra phase shifts that the extra output transistors introduce and keep the circuit stable. Does it say 6.8pF - 100pF? Are they suggesting you just try anything in that range?
Anyway, if the capacitor was way too big it would also roll off treble in the audio band.
The worrying thing about that capacitor is that the circuit could be unstable but with the oscillation in the ultrasonic band.

[Puffin]

Hi Andrew, without the 6.8uf caps all you get is oscillation (a whistling/signal type noise) I used a 7pf ceramic cap (6.8pf is the value screen printed on the board). It may be worth me trying higher values to see if it improves the treble, but without oscillation. Are they suggesting you just try anything in that range? This was an ebay buy with nothing other than the board sent to me and the schematic sent by email!

So far as C1 is concerned I will check the board. Because of the position I cannot now see the value. I will prob have to de-solder. Many thanks for your continued support.
Rob.