Discrete amplifier not passing audio

Question

I have a card that can receive two dual input opamp with feedback. The card is currently connected to V+ and V- only for taking measurements. The +24V rail is doing great. I also know for sure that both opamps are in good working order. One of them has received a full recaping job a few days ago. The receiving motherboard has also been fully recaped today.

When I connect the input and output audio cables I have no audio coming through (well very very low level grainy and distorted bits of audio).

I took a few measurements and some values are on the low side, while one of the pins gives me voltage where it shouldn't. The measurements are exactly the same for both opamps.

The measurements are as follows:

6 = 23.3V

5 = 12,2V

3 = 0V

1 = slowly dropping from 7V (instead of 0V)

7 = 8,9V (instead of 12V)

2 = 10,5V (instead of 12V)

I am attaching a few documents to help you understand the circuit better.

What would be your ideas? I can take measurements and replace faulty components but I must admit that I am not too at ease with that kind of opamp with feedback and decoupling..

Thank you!

Current setup (10k stereo pot at the bottom, output summing card at the top) :

Answer 1

The card is currently connected to V+ and V- only for taking measurements... I took a few measurements and some values are on the low side, while one of the pins gives me voltage where it shouldn't...

1 = slowly dropping from 7V (instead of 0V)

This is expected. Pin 1 is connected to the Base of TR1 via a 10 μF capacitor. If power has been switched off for a long time the capacitor will be discharged and have 0 V across it, so the initial voltage measured with a meter will equal the bias voltage on TR1 Base (~8,2 V). As the capacitor charges up through the meter's internal resistance a voltage will develop across it, reducing the meter voltage. Digital multimeters typically have a resistance of 10 MΩ when reading DC volts, which would cause the measured voltage to drop to 7 V in ~15 seconds.

Readings taken after power has been switched of for a 'short' time could start at a lower voltage because the capacitor is already partially charged. When the 10 kΩ 'gain' pot is connected to the input the capacitor will charge much faster and the meter will almost immediately read 0 V.

7 = 8,9V (instead of 12V)

2 = 10,5V (instead of 12V)

I simulated your circuit in LTspice. It calculated 10.6 V on pin 7 and 8.9 V on pin 2. There is no reason for pin 2 to be a higher voltage than pin 7, so I suspect you have the values swapped and the actual voltages are as expected.

The measurements are exactly the same for both opamps.

This strongly suggests there is nothing wrong with the 'opamps'. The DC voltages are also normal, so something else must be causing the weak distorted audio.

In the comments you say:-

The full circuit is as follows: XLR input > input transformer > 10K input gain pot > opamp > output amp with 10k gain pot > output transformer > output XLR.

and

I just plugged everything again (please check the pcb picture I posted for wire colors): output from 10k pot to white wire, ground from 10k pot to red wire, green wire to ground, blue wire to output

Your module pinout diagram shows input on pin 1 and output on pin 5, but the actual wiring has the white wire on pin 5 and blue wire on pin 1. So if your description is correct then you are feeding the input from the pot to the amplifier output, and taking the output from the amplifier input - in other words you have it wired backwards. In this 'reverse transfer mode' very little if any signal will get through, and it could be severely distorted as the amp fights against it.

Answer 2

Grounding -ve input to Vee=0V disables it.

You can use a differential input to Vin+ and Vin- but ONLY if the DC gnds and supplies are floating with respect to each other. Otherwise do not! Also do not put a cap across the Vin- to the rail. That will destabilize the internal feedback and it may oscillate. You can't AC ground the Vin+ and use Vin- either. This amp or the source must be "floating" and no use of SMPS either.

There are other methods, but this will work with 4 stages of gain being up 80 dB open loop with at least 20dB to 30 dB of AC/DC internal feedback.

It is a high gain low impedance current limited with fixed gain without Feedback added to reduce the gain but the internal 12V reference for the output and inverting input so and inputs and outputs must be AC coupled, but not Vin- which is low input impedance. That must be direct-coupled in differential mode to the inputs provided, yet floating. The input cap provided is to decouple from the inverting input ( the emitter) not isolate from the source.

The input level must be on the order of 1 mV and up and the source impedance determines the gain down to ohms for max gain!! So padding the source might be necessary for a large signal to a very low input impedance (< 8 Ohms for max gain) but I suspect <1K source impedance for reasonable gain is what it was designed for. ( I wonder what the user's manual says)

The lack of current sources everywhere with only 5 transistors relies on the high supply voltage for reducing the current modulation. So the output swing will likely be only a few volts max which is pretty loud for a headset.

Reducing the internal R feedback with the external FB connection to Output will reduce the gain very little (?). The feedback goes to the emitter so that attenuates the feedback with low impedance already, but current limited so it is intended for Sennheisers with high impedance. = 300 ohms not 8 ohms. With that load, I expect it should be high-quality THD ~ 0.1%

It is not your typical Op Amp design. "It's a NEVE"

• Your Voltage measurements are normal, but a more detailed explanation is needed to explain why) See my simulation for more accurate results

here is an accurate simulation of how to use the amplifier with a low impedance floating differential source and thus high gain.

You can add series source resistance (using thumbwheel over the 330 R source R) and experiment with observing the ratio of output to input gain with the output Vpp probe.

The schematic uses labelled Nodes to connect the inputs. Here the input is 1mVpp and the output is 2.134 Vpp with a clean undistorted gain of 2134. Vfb is left open.

Vfb (pin 7) is at a midpoint for constant current DC Voltage between Pin 2 (Vin-) and Pin 5 (O/P) but that ratio does not define unity gain. Rather the ratio of (R15+R4/ (Rsource/hFE)) emitter low input impedance defines the gain with both R5+R14 for AC feedback and DC self bias.