I need a bit of help - BMP only has unity gain

[Rain Dog]

I really need some expert help. I hope someone here has the time to work with me.

I built the BMP dern near exactly the way it is shown on the GGG site. I built it on a perf board and socketted the transistors. All transistors are 2n5089 that I have tested and they all seem to work and have hfe's in the high 700's.

I have tried two different signal generators and my real instruments and the results are the same. I even tried different scopes and probes as well as my amp. Virtually no gain through the system. I have measured an input 440 Hz signal at 350 mV p-p at the input to the circuit (plug). The signal after C3, which is after the coupling cap between stage one and two, is well under 100 mV p-p. How can this be??

I measured the R1 and it is 39K and not 39Meg or something so that's not it. I also put a pot in parallel with R1 so I could dial the resistance down to @0. The only way I can get even a small gain through Q1 is to dial the R1 resistance down to a few ohms. This result is independent of signal source.

All the transistors are on and seem to be biased properly. I was really careful with the resistor and capacitor values but obviously I have done something wrong.
Voltage Values: Battery = 8.8V
Q1 C= 3.9
  B= .62
  E= .033

Q2 C= 3.8
  B= .63
  E= .034

Q3 C= 3.6
  B= .62
  E= .023

Q1 C= 3.2
  B= 1.75
  E= 1.14

At this point I'm at a loss as to what to do next.

Thanks,
-RD-

[RDV]

Are you perfing? Or did buy or build a PCB? Have you got an audio probe? Cause that first stage should be amplifying right off the bat. C3 might be a bad cap?

RDV

[Rain Dog]

Are you perfing? Or did buy or build a PCB? Have you got an audio probe? Cause that first stage should be amplifying right off the bat. C3 might be a bad cap?

RDV

Umm, sooo, like I said.... I am building on a perf board not a PCB. I have access to a bunch of instruments so I tried my old tube based signal generator AND a brand spankin' new Hewlett Packard sig gen. I also tried two different dual trace O-scopes AND different probes AND I made an audio prob and stuck it on the end of my guitar cable AND that that to the amp.

I have tried the 5088's and the 5089's.

The gain is just not there. Tonight I am going to build just stage one on a proto board. If that circuit still doesn't have gain then ... I dunno, could the circuit be wrong? I did proto stage one a couple of weeks back and got no gain then either but at that time I only had the 5088's so I built the complete circuit with the 5089's when I got them.

Oh, and it's not C3 because I can get a trace BEFORE C3 and it is the same weak signal but with the DC offset. Also signal is getting through all the stages to the output jack.

I'll check back later...

[R.G.]

Quoting Sherlock Holmes to Watson "Eliminate the impossible, dear Watson, and whatever remains, however improbable, must be the truth."

Your bias voltages do look good. So we can probably accept that the resistors are connected to the transistor and are mostly the right values. So if the transistor is running about right, what else could be wrong?

(a) Something could be keeping the transistor from getting much input.
(b) Something could be keeping the amplified stuff from getting out
(c) The feedback parts could be telling the thing to run at a gain of one.

All three of these conditions can be affected by the capacitors. If C1 is internally almost open, or has a bad solder joint into it, or is way too small, you'd get (a). If C3 is bad in a similar way, you'd get amplification at the collector, but the signal wouldn't get past C3; that's (b). If C2 was way too big, you'd get (c).

With access to scopes, it should be easy for you to sort out which is which. Use a scope probe on both sides of each cap. Find out which one has substantially no AC voltage across it and which one has a lot.

[Rain Dog]

I vote "C"

I'll go through the feedback circuit tonight. "That's what I need, Clues, Mr Holmes!!"

[RDV]

If you need 470K resistors, I'm your man. I bought a 5000 reel off eBay on a whim(don't ask). A penny a piece is all I want. Got caps too.

Spam

RDV

[Rain Dog]

That's it!!!! Thank-You R.G.

It was the C2 Cap. I used 47nF caps in all the feedback loops when I thought I was using 470pF. I only had one 470pF on hand so I haven't gotten all the stages working but Q1 is kickin'. I have a shipment coming in from Small Bear so I will have this thing running soon.
'tis very good to have a forum like this!

RDV - I'll buy some of those resistors and probably a small pile of caps too. Do you have any 470pF caps?? I'll PM you soon. Now it's time for bed.

[RDV]

It's cool that you got it going, but I don't get how that would keep that section from amplifying. R.G.?

RDV

[R.G.]

I don't get how that would keep that section from amplifying. R.G.?

It doesn't keep it from amplifying - it sets the frequency response.

Here's the poop.

You think that Q1 is a transistor, right? It's not - it's an opamp.

Take the schematic of that first stage, remove Q1, the emitter resistor, the collector resistor, and the resistor to ground from the base. Now draw in an opamp triangle with output where the collector was, inverting input where the base was, and noninverting input where the emitter was. Ignore the DC levels for now.

R1 is the inverting input resistor, R8 is the feedback resistor, and C2 parallels the feedback resistor.  Ignoring C2, the gain is approximately R8/R1 or 12. At very high frequencies, C2 approaches the impedance of R1, and the gain falls from 12 to unity, and thens keeps falling because C2 approaches a short circuit.

With C2=470pF, the frequency where gain starts falling off is F = 1/(2*pi*470K*470E-12) = 720Hz. With C2=470nF, that drops by a thousand to .72Hz. Guitar notes from 82Hz to 656Hz (the three octaves of normal tuning below the 12th fret) are at unity gain - maybe.

Q1 is a feedback amplifier. Reducing the feedback resistor lowers gain, reducing the input resistor increases it.

[RDV]

That's what I thought(except not knowing the math of course). So he was EQing himself out of gain then?

Interesting

RDV

[Rain Dog]

Those transistor sockets have too sort of leads for me. They are hard to solder too.

I bought the capacitors at a small independant used electronics shop. I asked for 470pF and he handed me a bag of capacitors. I used them without measuring the actual value because the meter I was using didn't measure down to the pF range. My new meter does so hopefully that won't happen again.

R.G. - Big Thanks for the explanation!!

So.... RDV do you have any 470pF caps??