Serious Distortion on my DIY Toccata Phono Preamp

[bigpandahk]

After more than 100 hours run-in, the sound of my DIY Toccata Phono Preamp is still terrible. No bass, oweful mid-range and sharp terble. I suspected the RIAA caused the problem and setup the oscilliscope to check. I fed a 100mV 1KHz sine wave into the amp and got a square wave at the output. I checked the output of each stage and the waveform already distorted at the first stage. Then I disconnected the first coupling capacitor at point A (see attached diagram) and the output of the first stage is good. Can any expert advise me what's the problem?

I attached the circuit diagram and the two waveforms measured at point A with the couping capacitor to the 2nd stage connected and disconnected.

[DF96]

100mV input may overload an MM preamp. It will certainly overload an MC preamp.

You appear to be driving the second stage into grid current, which is causing the limiting on positive peaks. This is not surprising. Its grid bias is about -1V - about the level where grid current will start flowing for most valves. The 6GK5 is only intended for small signals, a few 10's of mV in a TV tuner, but you are feeding it many volts. Try repeating the test with 10mV input.

The RIAA bass boost is provided in this circuit by the 50K, 5K and 62nF after the second valve. It actually does the 'boost' by cutting everything else. Check these components. If they are faulty or the wrong values then you will get bass problems.

[banat]

Bidpandhahk try to run this phono preamp with adding some parallel capacitors with each 3V bias batery ,with value betwen 0,22uf - 1uf / 50V .

Best Regards

[bigpandahk]

Thanks DF96, I made a silly mistake again, will try 1mV input. For the RIAA, all the components have been checked again and found no problem. I disconnected the 910p and 62n capacitors and the sound only change slightly. Seems the RIAA is not function properly.

Dear banat, thanks for your advice and will try.

[DF96]

The 910pF does the HF rolloff above 2kHz so you should certainly notice if it is removed. Removing the 62nF will increase gain considerably, as well as changing the frequency response. If these have little effect then there is something seriously wrong with your system or your ears. Have you checked your resistor values with a meter?

[bigpandahk]

Hi DF96

I used 4mV input and the amp works fine with sine wave. Then I roughly checked the RIAA curve using the oscilliscope to measure the p-p output voltage and have the following result (with 4mV input):

Freq____O/P p-p____dB gain____ref to 1KHz
20______1.84______33.255______-3.892
50______2.28______35.117______-2.03
100_____2.36______35.417______-1.73
500_____2.7_______36.586______-0.561
1k______2.88______37.147________0
2k______2.64______36.391______-0.756
5k______1.68______32.465______-4.682
10k______1________27.959______-9.188
20k_____0.468_____21.364______-15.783

I believe the above result confirmed that there is serious problem on the RIAA EQ circuit. I have measured every resistors by multimeter and capacitors by LCR meter, found nothing wrong.

[DF96]

Yes, you have bass cut instead of boost. The circuit you have shown will not do that, so this means the circuit you have built is not the circuit you have shown. There must be either a component error or a wiring error.

Bass cut will come from either coupling capacitors being too small or load resistors being too small. You only have three coupling caps, so try checking them again or swapping them. Something to bear in mind is that you could be misinterpreting something, so you are now checking that the circuit is what you think it should be rather than what it actually should be.

You could also try checking the signal partway through the preamp. For example, at the second stage anode you should have the treble cut, but no bass boost or cut. This will help narrow down where the fault lies.

[bigpandahk]

Thanks again DF96. will disconnect each stage at the coupling capacitors and measure the frequency response. Hope can find what silly mistake I have made. May be I have to find someone else to check the wiring for me as I have checked it more than 10 times

[DF96]

Don't disconnect anything, as you might hide the fault that way. Just look at the signal at each anode, and the output. You should find it flat at the first anode, an HF cut (above 2kHz) at the second anode, and the full RIAA curve at the third anode and the output.

[bigpandahk]

Noted and thanks. Will report the result tomorrow.