ARC PH5 help

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This is a subject I'm moving from the Amplifier Tube section for discussion.

I have an ARC PH3 that I would like to update the the PH5 version and maybe change the RIAA to passive.

The question I have is where to put the RIAA filters. Looking at the schematic, I was thinking after Q5? I think ARC put one filter there?

Also since I don't have this circuit, I'm wondering what the current and voltage at R18 should be for Q5?


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I don't understand why you suspect that a passive RIAA will get better behavior than this using feedback, in fact it is not. In any case, to make a good passive network, the RIAA net need to be fronted with a low impedance source and high impedance load, to proper job unless you can include output and input impedances into the network itself, a task that requires deeper electronic analysis and knowledge.
If you want to update the PH3 to a PH5,sell the PH3 and buy a PH5.They are radically different designs.

If you want a unit with a fully passive RIAA network,buy a brand of preamp that incorporates this style of design from the outset.The PH5 uses a passive high/active low RIAA network.
If I wanted to buy or sell something, I would have posted in the swap meet section.

If I have to explain why I want a zero feedback phono stage, then you just don't get what I'm asking for.

I have over 50+ years in DIY (yep I'm a old fart), long before this site, back in the days even before Audio Amateur, or Glass Audio.

I'm a tube-o-holic. My current phono stage is a highly evolved, dual mono, CLC filtered, tube regulated PS, with mono, tube shunt regulated, real WE 417a input stage to a 01 DHT.
If I could get a DHT to be quiet enough, that would be my input stage (diff-amp???).

Copper foil/paper/oil caps (no aluminum foil or plastic caps please). Matched Carbon comp and WW resistors. The only place I deviate is for the RIAA which is passive, Vishay Z foil resistors and Teflon caps. Point to point Silk Silver OCC wire. Teflon/gold tube sockets.

The last store bought preamp I owned (long ago) was a ARC SP6, which I quickly gutted and added a tube regulated Joe Curcio "Daniel" design (passive RIAA). That was the last commercial preamp I've purchased.

The PH3 was part of a payment for a custom built, tube regulated, Marantz 7C clone, which BTW ran circles around the PH3.

The first thing I did to the PH3 was to remove the wimpy power transformer and SS regulator and replaced it with a Lambda tube regulator and Salas dual mono shunt regs for the input stage.

The PH5 suffers from the same wimpy power supply, and I will use the above PS. I just wanted to see what I could do with the Fet/diff-amp input stage of the PH5. I'm not a SAND guy, that's why I ask.

The Q5 Fet (I'm going to use a J74) is a low impedance source feeding a 1meg/tube high impedance 6DJ8 diff-amp second stage. I could just design Q5 with a 1K source resistor (front end is 18 volt regulated), and use an adjusted RIAA from the Salas Folded Simplistic.

I don't want to offend anyone, and just want to make it clear what I'm asking for.

Maybe I can get Salas to comment on that idea.
I miss-spoke above. I would adjust the Q5 DRAIN resistor (YEP not a sand guy) to get the proper voltage at the Q5 drain and then adjust the RIAA network for the proper values. As it now stands, I have the front end regulated in the PH3 to 18 volts with the correct resistors to drop the fet DRAIN voltage to 12.4 volts per the PH3 schematic.
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The PH3 is designed for MC cartridges, the levels at Q5 are going to be quite low and introducing passive EQ there is going to result in a significant noise increase. Q5 is a source follower and provides no gain at all.

The place to introduce passive EQ is in the tube section where the signal levels will be much higher. (After V1 plate, but you have to eliminate the negative voltage feedback to do so)

You are talking a total redesign of the tube portion of the pre-amplifier.
The question I have is where to put the RIAA filters. Looking at the schematic, I was thinking after Q5? I think ARC put one filter there?

If you really want to convert PH5 into passive RIAA, you can duplicate the tube output stage (V3 & V4), for the first gain stage (V1 & V2) and have the passive RIAA network sandwiched in between. Use an online RIAA calculator to figure out the component values and tweak for accuracy. You can re-purpose V2 into a cathode follower and Q6 as its current sink. Obviously delete the feedback. Essentially it's an MM phono stage in itself. And if you are not a solid state guy then you can replace the entire JFET input stage with a step-up transformer. It's a lot of work to mangle a PH3. Why not just build from scratch?


MM version minus the FET input stage.
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The PH3 uses only 3 tubes. To avoid adding new socket and keeping the similar topology to PH5, you need to not use two cascading tube gain stages before to the output cathode follower. As usual, take out the feedback and have passive RIAA network sandwiched in between two gain stages. The FET input stage, however, will invert absolute phase.

All good ideas! Kind of want to listen to the fet diff-amp?

I think I will leave R20 & C6A+B and then add the low freg network filters after the tube stage. Not sure if I'm going to change the PH3 cascode to the PH5 tube dif-amp CCS? Any thoughts?

I may build a new board (or PTP) and remove the stock one. Already did that with the PS.
In the PH5 schematic, isn't R20 and 6A+B//C the high freq RIAA filter? I just don't want to duplicate it. Seems 6A+B//C value is off? According to the KAB RIAA calculator, it should be
.015030 uf?

The KAB calculator has to be used with a total of FOUR components to be accurate. You cannot separate them independently.

The stock PH5 circuit uses two separate RIAA filters, R20 and C6 are for the high frequency roll off. And the feedback is for bass boost. So PH5 uses really a hybrid RIAA,half passive and half feedback. That's why R20+C6 are deleted in the modified schematic to be able to use KAB's all in one filter.

To avoid confusion, I'd suggest simply follow the stock circuit and be done with it.

I have built several split RIAA phono stages, including the Daniel, Siren Song, a few Allen Wright's designs and a Italian ??? one. They were some of the best phono stages I've had the pleasure of building and listening to.
I don't expect the PH3 or PH5 to compete with those. This is just a test of what could be done to the PH3 to make it passive, without a total redesign.
I'll have to dig out all my old info on split RIAA. As long as I get the turn-over points right I should be 90% there.

Thanks everybody for you help.
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