I unplugged the leads from the main amplifier and measured resistance btween the two centre pins: it's reading ~ 3 ohms (so definitely continuity there).
3 ohms is low but way above what a direct piece of wire should read. If you short your meter leads out then you should read 0.00. Anything above that is a combination of DVM error (that should be miniscule) and the resistance of the test leads. Typically they might be around 0.3 ohms for an average 'hobby' type meter.
So... you have that PLUS the resistance of the signal leads and your shorting links in the preamp. It just sounded a bit high but maybe they are thin audio leads.
If we accept the continuity is good, and that the signal is identical on both leads then the imbalance you are hearing up to now must be external to the preamp.
So... you have that PLUS the resistance of the signal leads and your shorting links in the preamp. It just sounded a bit high but maybe they are thin audio leads.
If we accept the continuity is good, and that the signal is identical on both leads then the imbalance you are hearing up to now must be external to the preamp.
Hm that is strange. Yes the leads read 0.00 touching each other.
The wire is 20 AWG from socket to pcb. However for doing all this testing I have been using a breadboard and jumper pins, which seem to be very thin cables. I am going from the breadboard to the sockets with those pins, which maybe could account for the difference?
So does this prove that U1 is completely ok? What would be my next step in figuring out what's going on with the right channel?
The wire is 20 AWG from socket to pcb. However for doing all this testing I have been using a breadboard and jumper pins, which seem to be very thin cables. I am going from the breadboard to the sockets with those pins, which maybe could account for the difference?
So does this prove that U1 is completely ok? What would be my next step in figuring out what's going on with the right channel?
Are we saying the 3 ohms is down to the wiring and breadboard then ? Looks that way.
All we have been doing with these last tests is proving (or disproving) that the imbalance you hear at this point must be external to the preamp. It has to be, because you are feeding the same signal to both sockets.
Is U1 OK? Well I think it probably is. If it wasn't amplifying then you wouldn't hear anything given that its a phono cartridge feeding the input.
We can check by you going back to the diagrams in post #44 and for you to move the red common connection from U1 output over to U3 output. That repeats the last few tests but this time by using U3. You should hear the same kind of levels.
If that's OK then we move on to refitting the missing opamps and then logically trying to trace the signal from those opamp outputs. One at a time if need be.
All we have been doing with these last tests is proving (or disproving) that the imbalance you hear at this point must be external to the preamp. It has to be, because you are feeding the same signal to both sockets.
Is U1 OK? Well I think it probably is. If it wasn't amplifying then you wouldn't hear anything given that its a phono cartridge feeding the input.
We can check by you going back to the diagrams in post #44 and for you to move the red common connection from U1 output over to U3 output. That repeats the last few tests but this time by using U3. You should hear the same kind of levels.
If that's OK then we move on to refitting the missing opamps and then logically trying to trace the signal from those opamp outputs. One at a time if need be.
@Mooly, before I try that, is it possible I can test if the right channel audio signal is making it from the input socket to U1, using a multimeter?
I have a hunch maybe something is off very early on in the path.
I have a hunch maybe something is off very early on in the path.
Yes. Look at the circuit. The input goes directly via a 2k2 into the opamp input. So you should read exactly 2k2 from input to pin 3 or pin 5 depending which channel you are measuring however to get a true reading you will need to remove the opamp. Having the opamp present may cause an odd reading due to interaction with other components.
That will only prove the continuity. It wont prove the stage is working.
That will only prove the continuity. It wont prove the stage is working.
Ok, do you mean 1k resistor after the input , not 2k2? I have a 1k resistor there which is what I'm looking at on the Rod Elliot diagram.
I tested from the input socket L channel to U1pin3, and input socket R channel to U1pin5.. both correctly give me 1k ohms. So this shows the continuity is correct, but how would I prove the stage is working?
Also, in your post #64, you mentioned U3, but I see no U3 on the diagram - did you mean something else?
I tested from the input socket L channel to U1pin3, and input socket R channel to U1pin5.. both correctly give me 1k ohms. So this shows the continuity is correct, but how would I prove the stage is working?
Also, in your post #64, you mentioned U3, but I see no U3 on the diagram - did you mean something else?
This is the page from Rods site that I'm working to:
Hi-Fi RIAA Phono Preamp
The simulation I did for you shows both channels, U1 and U2 for one, and U3 and U4 for the other. The series input resistor is shown as 2k2.
You need to be able to think it through for yourself 🙂 and to understand the logic of the tests.
How to prove... well all we have been doing is the equivalent of you taking one channels input on your main amplifier and using that to probe the preamp. With grounds correctly connected you can probe all four opamp outputs and listen to where the signal is missing. That's really all we have been doing.
Ideally you should have a test signal connected to the preamp inputs, and use a 'scope to view the signal at all the points in the circuit.
Hi-Fi RIAA Phono Preamp
The simulation I did for you shows both channels, U1 and U2 for one, and U3 and U4 for the other. The series input resistor is shown as 2k2.
You need to be able to think it through for yourself 🙂 and to understand the logic of the tests.
How to prove... well all we have been doing is the equivalent of you taking one channels input on your main amplifier and using that to probe the preamp. With grounds correctly connected you can probe all four opamp outputs and listen to where the signal is missing. That's really all we have been doing.
Ideally you should have a test signal connected to the preamp inputs, and use a 'scope to view the signal at all the points in the circuit.
Ok, I see I think that diagram is an older revision. When you purchase a board from his site he sends you a secure link with the latest information, and that one has a 1k instead of 2k2 resistor. 
I see I was confused about the U3/U4 because it's not on the Elliot project diagram. But you were just using that as the "right channel" version of the left channel diagram you drew, to make it easier to separate the two?
Anyway, I can see if I can find a friend with an oscilloscope and signal generator. Can I only continue the testing if I use those?
I see I was confused about the U3/U4 because it's not on the Elliot project diagram. But you were just using that as the "right channel" version of the left channel diagram you drew, to make it easier to separate the two?
Anyway, I can see if I can find a friend with an oscilloscope and signal generator. Can I only continue the testing if I use those?
use a DMM set to Vac with a signal generator.
There are computer programs that mimic signal generators.
There are computer programs that mimic signal generators.
Its the best way. There are cheap USB scopes available that work with your PC (I've never used one myself) and although they are limited compared to a 'proper' scope, they should at least allow audio signals to be displayed accurately.
You can make a test tone either as an MP3 file, or burnt to CDR in seconds using Audacity.
Yep, just waiting on my oscilloscope (I ordered an old analog one). For a signal generator I'll just use my DAW - I can output any sine wave.
Excellent 
That should help nail the problems, and an older analogue type is by far the best for this kind of thing .
That should help nail the problems, and an older analogue type is by far the best for this kind of thing .
That's what I read 🙂 It's a Tektronix 422, but apparently the intensity knob is a little finicky. Hopefully it should work for what I need though.
Excellent
That should help nail the problems, and an older analogue type is by far the best for this kind of thing .
Excellent
That should help nail the problems, and an older analogue type is by far the best for this kind of thing .
Hi Mooly, still alive here😀
I finally got my hands on an oscilloscope, although not an analog one. It's a handheld multimeter/scope.
Anywhere here's the readings I'm getting, with both U1 and U2 installed:
I'm using a program to output a 1khz square wave test tone through my audio interface and into the phone inputs (both L+R) on the preamp.
For the scope it's on the "Hz" setting. I've connected the black/ground probe to common ground on the case, and then taken readings with the red probe.
U1
pin 1 (Out L) : 1khz / 154mV
pin 2 (- In L) : 0hz / 46mV (says 0hz but I do see a square wave😕)
pin 3 (+ In L) : 0hz / 46mV (says 0hz but I do see a square wave😕)
pin 4 (-VE) : 0hz / 12.3V
pin 5 (+ In R) : 0hz / 19mV (see straight lines up and down in even intervals, but no clear wave)
pin 6 (- In R) : 0hz and no mv reading. Looks like a garbled messy audio wave
pin 7 (Out R) : Same as pin 6
pin 8 (+VE) : 0hz / 12.5V
U2
pin 1 (Out L) : 1khz / 4.8V
pin 2 (- In L) : 1khz / 155mV
pin 3 (+ In L) : 1khz / 155mV
pin 4 (-VE) : 0hz / 11.87V
pin 5 (+ In R) : 0hz / 0mV (garbled looking wave form)
pin 6 (- In R) : 0hz/ 0mV (garbled looking wave form)
pin 7 (Out R) : 0hz / 9mV (random looking garbled wave form)
pin 8 (+VE) : 0hz / 12.5V
So... appears the audio is never properly getting to U1 in the first place?
That's good.
We need to clarify a couple of things here in order to get meaningful readings.
Firstly we should be testing with a sine wave rather than square. The RIAA equalisation will make a square wave look horrible, and its also harder to tell if a squarewave is distorting or clipping.
So you need a sinewave.
You also need to be sure you understand the scope and how to interpret the display and the amplitudes.
So the first thing you need to do initially is forget the preamp and concentrate on the sinewave generation and reading it on the scope. You need to generate a 1khz sine that will have an amplitude of around 50millivolts peak (100mv peak to peak). The exact amplitude isn't critical as long as we know what it is and that it is somewhere in that amplitude region. We need the amplitude high enough to bring things out of the noise and yet low enough that it will not overload the preamp.
Lets get a ball park figure......
A CD player outputs approximately 2 volts RMS at maximum (0db) output. That is 2.8 volts peak. We want around 50mv peak so that is around 56 times less.
I've attached a test track that should be in the right ballpark. Its a two channel (stereo) MP3 1kHz sine of around -35db that plays for 5 minute.
If you connect this to the scope then you should see a nice clean sine wave. Nothing else will do.
You tell me what amplitude you see it as on the scope.
1kHz TEST TONE
Mooly, I wanted to say thank you again for your help in this thread. I had abandoned this over a year ago and got busy with other things. I pulled this out recently and decided to have another go with a fresh look and a cup of tea. Also I finally got my hands on a proper scope (tektronix 475, ugly thing but it works great and cost me $100).
I verified all stages across U1 and U2 were working properly... however it looks like my problem is at the input connector. I really don't know how I missed it, but I think I had assumed the input connector pins left and right were symmetrical - they weren't (verified with the scope). So the pin carrying the right channel audio signal was actually never getting to the board. I will try to fix tonight and verify that it's all working once put back together.
Sometimes it takes a while to get to the simple answer 🙂
sometimes taking a break from a 'problem' allows you to see things differently when you return to it. That has happened to me before now.Good luck, lets hope it all works 🙂
Good luck, lets hope it all works 🙂
It works!
thank goodness for that - Status
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