78RPM Heaven - a discrete multicurve phono preamp for your shellac! - diyAudio
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Old 24th May 2014, 05:09 PM   #1
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Lightbulb 78RPM Heaven - a discrete multicurve phono preamp for your shellac!

Hi everyone!

Here's the circuit I plan to build for casual 78RPM listening. I've designed it so that it gives a good sound for all of my discs for straight off the disc listening. It features a 3 pole rumble filter set to 35Hz a low pass filter set to 10KHz and all the eq. options you'll ever need.

I got creative with the topology as it's been a while since I built a discrete design, as this isn't a distortion critical application (although the distortion performance should be fine) I decided to build an all discrete phono preamp for this purpose.

The front end is a BJT cascoded low noise JFET (2SK170) gainstage with a flat gain of 37dB. The bias is set with a potentiometer (to 14V for the most headroom) as I've found the variation in drain current with these FETS to be a little to great to go without.

This is then fed into a low pass 10KHz 3 pole Butterworth filter, this may seem a bit extreme but the vast majority (97% from my restoration experience) of my discs don't have a true response past this point and there's some pretty nasty noise past that point anyway. This then feeds into a 3 pole high pass Butterworth set to 35Hz, once again, I've very rarely seen any information on the spectrogram that wasn't rumble past this point so it's a good idea to get rid of this part of the spectrum too . Both filters are based on BJT emitter followers loaded with JFET current sources. The filters may not seem necessary but the simple fact of the matter is that there really isn't anything worth listening too without noise reduction past this point and with a flat eq (common for pre 1950 pressings) the gritty HF portion is pretty unpleasant and distracting. The same can be said with low frequency noise. 78s are generally not 'hifi' so what we're after is the most pleasant sound possible . It's only common sense

The high pass filter feeds a 6dB low pass filter which is used to apply the treble cut part of the correct eq. curve. Pretty straightforward.

The bass boost is done using an active shunt feedback stage in the usual way. The collector load of Q7 is bootstrapped to increase gain (and thus linearity). R24 and R25 set the output bias voltage to about 14V. This is then followed by Q8 to the output which is decoupled by C18. Feedback is taken after decoupling to improve frequency response (it doesn't make too much of a difference but it saves a capacitor that would be used to separately decouple the feedback network). The output stages load driving ability is improved by configuring it in a similar way to the 'White cathode follower' circuit popular with tube designers. As the circuit is mono the load is going to be a parallel of two stereo inputs to the main amplifier. This will present a minimum load of 5K so rather than increasing the current of the CCS (which would have made it less linear) a couple of resistors and capacitors allow it to be modulated to improve the efficiency of the output stage while also slightly improving the linearity of the follower. Win win

So there you have it. A good way to listen to your 78s in all their warm crackly glory on your top notch HiFi. I'll be building the circuit within a week and will post pics plus audio snippets.

It runs off a 24V supply. I'll post schematics of that and the muting circuit tomorrow.
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File Type: png Full-78-Preamp.png (67.5 KB, 326 views)
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Last edited by monty78pig; 24th May 2014 at 05:11 PM. Reason: Forgot to attach schematics
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Old 25th May 2014, 02:56 PM   #2
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Arrow Improvements + power supply and muting circuit

Here's the circuit + a few improvements. I've raised the supply rail to 28V (as there's no point wasting the excess created with a 24V transformer especially when it could be used for extra headroom). I've adjusted a few biases to take advantage of this. The first gainstage can now do 10V peak to peak. More than enough for a MM cartridge with gain taken into account.

The power supply is a simple LM317 type with improved ripple rejection via C23 set to 28V.

The muting circuit contains a simple discrete 12V regulator to drive the muting relay. Fairly straightforward, a divider checks the voltage across the pre regulated part of the power supply, when the voltage is higher than about (while the preamp is on) 30V Q12 steals current from the base of Q13 causing it to stay off. This allows current to flow into the base of Q11 creating a reference voltage for Q10 to follow opening up the relay and illuminating the power indicator LED. C21, R40 and D2 make up the necessary time constant (about 6 seconds or so) which makes sure that the circuit has time to stabilise before the output is ready. When the supply voltage drops as the circuit turns off Q12 turns off in turn causing Q13 to conduct and shunt current away from Q13 while quickly draining the C21 so that it the preamp should be turned on again rapidly then it will give the audio circuit another 6 seconds to stabilise. This ensures that under no condition will turn on noise ever be heard .

I've modified the shunt feedback stage slightly to allow better approximation of the eq. curves while keeping the low frequency response predictable.

That's all for now. Time to start building
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File Type: png 78RPM-Heaven (1).png (107.4 KB, 289 views)
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Old 25th May 2014, 03:02 PM   #3
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for efforts and sharing

though ...... I don't have any of 78 discs
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Old 25th May 2014, 03:07 PM   #4
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Thanks Zen Mod, not many people do really so not really expecting any replies. Just want an online record TBH, if one other person builds it then it'll be worth the trouble.

Having said that quite a few people are frustrated with the cost of commercial ones, so who knows?
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Old 25th May 2014, 03:11 PM   #5
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nice to know that someone is really bothering with listening , building for and restoration of these gems

having some experience with capture and sound editing ( in previous life ) , I'm able to appreciate passion and energy involved in
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Old 25th May 2014, 03:16 PM   #6
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Yeah, if you want to then check out some of the stuff I do on soundcloud and youtube...

https://www.youtube.com/watch?v=VES_ez8lU1c

https://soundcloud.com/monty78pig

Gonna be uploading some more soon
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Old 25th May 2014, 08:53 PM   #7
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Old 20th June 2014, 01:21 PM   #8
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Arrow Minor modifications - boards made!

Finished building the veroboards, will post pics later.

Also made a few modifications to the equalisation network.
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File Type: png 78RPM-Heaven (2).png (112.2 KB, 211 views)
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Old 20th June 2014, 01:58 PM   #9
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2N5459 is a bit difficult to source -- the surface mount version is readily available, however.
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Old 20th June 2014, 03:03 PM   #10
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It's not super critical. Any 3.3mA current source will do.
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