Neve BA283AM questions(s)

Thanks Ian!

George

ruffrecords said:

Yes and yes.

Cheers

Ian

Click to expand...

Hi again,

So I have bread-boarded 1 CH of the circuit, but I am having some issues. A link to the schematic I am working from (it wouldn't let me attach anything else here) is here:

http://www.spattaweb.com/Amp_3.PNG

I am not using the input transformer at this point, I wanted to make sure the amp was working properly first.

I am feeding a ~100mV RMS 1kHz sine wave into the base of the 1st BJT with negative side going to common (J).

I am not seeing anything at the emitter of the 1st BJT. I should be seeing a slightly amplified input signal correct? I traced the signal through the blocking cap & series resistor to the base & all is good. C3 is good as I am seeing the input on the base side, and VCC on the other side. When probing the emitter I get nothing, no noise or anything. I have triple checked my wiring & can't figure it out. I also see the input on the base side of C7, and nothing on the other side.

I am seeing +24VDC on Pin 1 of the output transformer, but am only seeing about 480 mV on the collector of the 3055 and Q1. I'm measuring about 1.6V on the collector of the 1st BJT.

I'd appreciate any thoughts!

George

The voltage on pins 1 and 4 of the transformer should be almost the same - there is very little voltage drop across it. You should adjust RV! intitially for about 1.5V across R7. Sounds to me like the 2N3055 collector is not connected to the transformer.

You will see a signal just a little less than the input signal at the emitter of the first transistor if it is working correctly but with with C8 and R9 in circuit the gain is boosted to about 33 times so you should get about 3.3V rms at the 2N3055 collector.

Cheers

Ian

Hi Ian,

I definitely have something wrong here. I get 24V at Pin 1 of the transformer, and about 400mV on Pin 4. The collector of the 3055 is definitely connected to Pin 4.  I am not seeing anything across the 47 ohm resistor. I have tore the circuit apart and rebuilt it twice very carefully. I checked the BC184Cs out of circuit & they were good. I don't know what else to do besides tearing it down and starting over again.

The schematic that I linked to in my last post looks correct to you?

Thanks again!

George

So instead of ripping everything apart for a 3rd time, I ran the circuit in simulation, and came up with basically the same results that I was having on the bench. So something is amiss with the circuit itself.  You can see that there is a little less than 1mV on the emitter of the 1st BJT, and what the sim shows  on the transformer primary is also about what I'm getting.

See simulation here:

http://www.spattaweb.com/Amp_3_Simulation.PNG

Thoughts?

Thanks,

George

What is the dc voltage at Q3 collector?

Cheers

Ian

Hi Ian,

I am measuring 1.183 VDC. Simulations says about 1.2 VDC (the transistor models I am using are not exactly the same).

Thanks again!

George

Gsrokmix said:

Hi Ian,

I am measuring 1.183 VDC. Simulations says about 1.2 VDC (the transistor models I am using are not exactly the same).

Thanks again!

George

Click to expand...

OK, so Q3 is pretty much off but Q1 and Q2 do not seem to be conducting.

The reall puzzler is how you manage to have nearly the whole 24V supply across the transformer primary. What dc resistance do you have for it.

By the way it is not meant to be a 1:12 ratio; maybe 1:2 but not 1:12.

Cheers

IPS. What kind of sim are you doing to get these values?

Hi Ian,

I am using Multisim.  Tying A to ground for this circuit was ok? That cap and resistor from +24 to ground is ok? I thought I made the output transformer model a 1:12 because  on the datasheet I read to get 10dB of gain it was a 50 to 600 ohm wiring of the primary to secondary.

Thanks,

George

Maybe the variable resistor is the problem. Can you try setting it to 100%?

Cheers

Ian

Hi Ian,

I tried changing the value of the pot with no luck. I did remove the cap & resistor that were tied to Pin A in the original circuit, and now I am seeing 1.55V on Pin 4 on the transformer. Am I getting closer? Simulation attached.

Thanks,

George

Gsrokmix said:

Hi Ian,

I tried changing the value of the pot with no luck. I did remove the cap & resistor that were tied to Pin A in the original circuit, and now I am seeing 1.55V on Pin 4 on the transformer. Am I getting closer? Simulation attached.

Thanks,

George

Click to expand...

Are you using a Spice ORCAD??....please send a copy of this file in order to test it here maybe can help!

Opacheco.

You should not remove the 47 ohm resistor; this is what creates the bias. Try taking out the variable resistor and replacing it with a pair if 2K2 resistors.

The big problem you have is the dcr of the transformer should be very low so the voltage at pin4 should be very nearly 24V. What is the dcr of the transformer you have included?

Cheers

Ian

Opacheco, I do have an ORCAD file. I will upload to my FTP site this evening & include a link to it.

Thanks

George

Ian,

The DC R of one parallel side of primary is about 8 ohms, and 6.7 or so ohms on the other parallel side. I am getting the full 24V from the PS on the transformer primary. I am only using 1 parallel primary input for my signal at the moment, and have the 2ndary wired in series with the 1.5k R & .01uF cap across pins 5 & 8. I have other 4.7k trimmers I will try. I have put the 47ohm R back in.

I am at school now, but will check it all again later when I'm back in front of it.

Thanks again!

George

Just to give a feel for how the bias is supposed to work, if TR1 (in the Neve schematic) id off then its collect will try to rise towards 24V due to the 68K collector load R3. This will start to turn onTR2 which in turn, turns on TR2. The current flowing ht e the 47R resistor R7 develops a positive voltage, a fraction of which is tapped off by RV1 and fed back to the base of TR1  via the 56K resistor R2. This tends to turn on TR1 so is draws more current so the voltage across R3 drops and TR1 and TR2 become less turned on. In a very short time it settles down to a fixed current in through the 47R resistor depending the setting of RV1.

None of this works unless the collectors of TR2 and TR3 are connected to the 24V supply which they are via the transformer primary which has a very low dc resistance. This means that at all settings of RV1 these two collectors should be very close to 24V. This is what puzzles me about your sim and is why I think the problem may lie in the simulated transformer or the simulated RV1.

You will notice also there is no way the base of TR1 can have a negative dc bias yet your sim seem to think it has. Another reason to suspect the sim.

Cheers

Ian

Hi Ian,

Thanks very much for explaining how the circuit works. It all makes sense. While replacing the pot with another to try, I saw that it was in fact wired incorrectly. I was 1 column off on the breadboard (I had checked all of my connections several times, but apparently missed it every time), which was essentially shorting the emitter of the 3055 to ground. No wonder why it was getting so hot!

Anyway, I seem to have it sorted now. With 100.9mV in, I am getting 1.7V across pins 5 & 8 of the output transformer. The signal looks nice and clean on the scope.

Can you please tell me how to properly adjust RV1? I have it set roughly in the middle, and I do not see any difference on the output waveform while adjusting it.

I was so confused about why the circuit wasn't working that I ran the sim just to get an idea of what voltages I should have been seeing, at different points. I completely agree with you in that the model of the transformer I was using, was not similar to the actual transformer I am using. This is an academic exercise for me, and I am indeed learning a lot while trying to build this project.

Thanks again!

George

Glad you got it working. Subtle mistakes are very rare and this case was no exception.

RV1 sets the standing current in the 2N3055. This needs to be sufficient to meet the performance goals of the design (+26dBu into a 600 ohm load) but low enough that the 2N3055 does not get too hot. At normal levels into normal loads you will not see any noticeable change in the output waveform.

From memeory the bias current need to be in the region of 40mA to 60mA which represents a voltage of between 1.88V and 2.82V across the 47R resistor. I think if you search on line you will find a detailed set up procedure (not something I ver had to do when I was at Neve)

Cheers

Ian

Yeah what a relief! So I wanted to clean up the breadboard & shorten all of the leads of the components because I need to bring it to school to demo for the professor, I only pulled one component at a time so as not to make a mistake, but now that I have everything back together, I am getting a weird oscillation from the 3055. When I touch the collector (case) with my finger, I now have a Thereman going on instead of my mix amp! I noticed that when I touch C2, the same thing happens. I have gone through the circuit a bunch & can't find any wiring issues this time.  Any thoughts?

I tried replacing C2, and then the 3055, and then also put another C3 with the long leads back in, but I am still getting a high freq. oscillation that I can hear with my ears from the 3055 on the breadboard. I'll attach a picture so you can see the layout.

Thanks,

George

What are you powering it from? How far away from the board is it?

Check all the small caps around the first stage are properly connected, particularly C2, C3 and C5.

Cheers

Ian