Stereo Phono Cartridge to both Stereo and Mono Amps

It took a bit longer than expected, there's a lot of BC109's! 

The image below shows just the left channel, I will copy-paste it to make the right channel once this one is reviewed. There is a PDF version of the circuit which may be easier to see.

I wanted to highlight/ask these things:

1. I've only used through-hole parts, so the board might end up being perhaps eurocard-sized, but as I understand there is room for that. 

2. I've added a Line input too, since it's low-hanging fruit. If it's not needed, then the footprint for the relay contacts can be easily jumpered with wire, since then the relays (one for each channel since they are inexpensive and will simplify the layout) are not required.

3. The mono output is perhaps a few hundred mVp-p when the main output is say 2V p-p, since the mono input will be expecting a low amplitude, but I don't know the precise value, so there are trimmers for that, labelled VR6 on the left channel.

3. It needs an approx 24V supply, I'm thinking I could add a bridge rectifier if desired so that it can connect to a transformer. What's the opinion on that? 

4. Considering adding a couple of small solder-in fuses on the mono output connections (both signal and ground) in case some problem occurs with the vintage equipment; is this a good idea, or not necessary?

I have to say that I really don't "rate" the luxury version of the circuit - its way to complicated and very 5 decades ago.

It also uses a very supsect and noisy technique of running the ceramic pickup into a low impedance with two pots and attenuation on the input signal for maximum noise. The 50k pot is an impedance matching effort. But the net effect of all this is to introduce a first order high pass filter with a -3dB point at about 1kHz and then recover the response with the frequency compensation in the first stage amplifier. That is pretty yucky.

I would go for a high input impedance op amp buffer or possibly a charge amplifier with a first order high and low pass filters set to approx 50Hz and 15kHz. Equalization can be added once the signal is buffered and at a reasonable impedance level.

I'd expect the cartridge to have about 100mV ouput (won't know untill I test) so even a fairly rough op amp with 10nV/rtHz noise will give a signal to noise ratio of 98dB - which is way better than the vinyl.

Fine tuning the equalization means using a frequency response test record (if you are willing to sacrifice one to a ceramic pickup) and measuring the arm cartridge resonance frequency and Q - or else do it by ear !

I don't quite understand Q6 and Q7 in your circuit.

The 220nF across the emitter load resitance will just introduce a lot of distortion at higher frequencies.

Why not mix the two channels at the mix ballance pot (feed once channel into each end via 22k) and have just one buffer amplifier.

MK

Hi Michael,

I see what you mean. The charts below show the response measurement of the first stage (I can't simulate the entire circuit end-to-end due to a license limit).

Due to the low input impedance they are setting the gain to more than 20 dB higher to compensate at the low end. With the 1nF model, the response then becomes levelled out with the input trimmer set to approx 40kohm. It doesn't seem an ideal way to interface to such a cartridge : ( I'll put this design on hold since it has these shortcomings, and it won't perform like a modern circuit. I guess magnetic cartridges superseded the ceramic ones and no-one improved these circuits much, since there doesn't seem to be many designs on the web.

Regarding Q6 and Q7, they were there to reduce load on the main stereo output, because the simulation showed a bit more distortion with a lower resistance for the load. I could increase the mixing resistances and replace with a single amplifier, but since it was just a single extra BJT I left it there since I'd already simulated with that topology. I didn't see much distortion for that section with the simulation, it was pretty negligible at that low output amplitude for the mono output, but perhaps I should have simulated with a higher input. I was just guessing what the cartridge level would be at that time.

Hi Michael,

I see what you mean. The charts below show the response measurement of the first stage (I can't simulate the entire circuit end-to-end due to a license limit).

Due to the low input impedance they are setting the gain to more than 20 dB higher to compensate at the low end. With the 1nF model, the response then becomes levelled out with the input trimmer set to approx 40kohm. It doesn't seem an ideal way to interface to such a cartridge : ( I'll put this design on hold since it has these shortcomings, and it won't perform like a modern circuit. I guess magnetic cartridges superseded the ceramic ones and no-one improved these circuits much, since there doesn't seem to be many designs on the web.

Regarding Q6 and Q7, they were there to reduce load on the main stereo output, because the simulation showed a bit more distortion with a lower resistance for the load. I could increase the mixing resistances and replace with a single amplifier, but since it was just a single extra BJT I left it there since I'd already simulated with that topology. I didn't see much distortion for that section with the simulation, it was pretty negligible at that low output amplitude for the mono output, but perhaps I should have simulated with a higher input. I was just guessing what the cartridge level would be at that time.