Output impedance of p10

[rahman]

Holas

Tossing up whether to give one of those LDR passive jobbies (e.g EVA or lightspeed) a go with my system and, from what i've read, they work best with low output impedance of source (circa <100ohm) and high input impedance of amp (circa >50k).

It's been hard to track down the reported output imp of the p10 phono. I've read somewhere it's around <5k, but that was from a review (so grain of salt taken). Can anyone confirm?

Also, anyone got any first hand experience of the LDR jobbies with a SS muscle amp to share? Rest of my system should be in sig.

Cheers
Adrian

[Barry]

Hello Adrian,

I'm not sure if by a 'P10' phono amp, you mean the BAT VK-P10 design. If you do, then owing to its high output imedance of ~ 1.9KOhm, it may not work well with a passive control unit.

Regardless of whether the passive control unit uses a potentiometer or LDRs, it will have a resistance range of 0-10KOhm. When used at roughly the mid-point, the effective source impedance as seen by the following amp will be ~2.9KOhm. This means that ideally the input impedance of the following amplifier ought to be at least 30KOhm, if not greater.

I note that you use an EMT 930st. Is this equipped with the EMT 155st equaliser/amplifier? This has an output impedance of 40Ohm, so will be fine with a passive control. Since you do not mention any other source, why are you using the Bryson preamp? Why not run the output from the 155st into the passive control unit and thence into your Bryson power amplifier?

Regards

Barry (fellow EMT user)

[rahman]

Hi Barry

Thanks for your input.

Just to clarify, I'm using a puresound p10 phono. Sorry, should have been more clear there.

I'm using the p10 instead of the 155st as I've found the p10 removes the SS edge/glare from my system, particularly in the highs, and provides a bit more detail as well. That said, not too shabby for 40yr old+ electronics design by EMT...

The p10 runs into the bryston, which largely serves for HT duties, but also has a decent analogue stage. As I currently have too much gain with the p10, I thought a decent passive attenuator would bring me a step closer to the source by side-stepping the sp1.7 altogether.

I guess I could try it out with the 155st and see if the upsides of bypassing the 1.7 outweigh switching away from the p10 - but in the longer term I'll prolly be running 3 TTs and will need a separate phono for probably 2 of them (and want to have my cake and eat it too..)

In terms of your math, is the figure of 2.9K based on the source imped of 1.9K (for the BAT) or more general? The input imp of the 14B is 50K/58K un/balanced.

Cheers
Adrian
ps: good to see another EMT user. which one are you running?

[Barry]

Hi Adrian,

From what I can find out about the Puresound P10 phonoamp, it uses an anode follower, so its output impedance will be high; the ~ 5kΩ you quote is quite plausible. Puresound is a member here, so perhaps he can help you with your enquiry, or phone him on 01822 612449.

You could always 'cut the Gordian knot' and make up a passive control and try it out. All you need is a twin ganged 10kΩ (logarithmic) pot. They are not expensive: a twin ganged carbon track pot can be bought from Maplin for less than £1. If it works well, then you can consider replacing it with say a Bournes conductive cermet track device, or an Alps thick film discrete step attenuator.

Should you choose to go down the LDR path (and I must admit it does look like an elegant solution), Silonix make an integrated LED/LDR device: the NSL 32 SR2. Bear in mind that owing to manufacturing tolerance, you will have to buy about 10 of these devices so as to be able to select two that track closely enough for stereo use.

I use an EMT 930st deck with EMT929 arm and a TSD15 (SFL stylus). In this configuration I use the EMT 155st equalising amplifier. I also have two EMT XSD15 cartridges that I use in either an SME 3009 or a 3012 arm and can also use the TSD15 in either of these arms with the Ortofon Neumann/EIA adaptor.

Regards

[pure sound]

It is 5Kohm which is high but into 50K or higher not a problem. Keep the interconnects to a sensible length.

Apart from using output transformers I find the hoops that must be jumped through to reduce the output impedance from a valve stage are an unacceptable compromise (cathode followers & suchlike)

Paralleling output valves would bring a reduction but even that isn't without cost and also asks more of the power supply.

Use a 50K or a 100K pot on the input of an integrated amp or pre with buffered output and it will be fine.

[anubisgrau]

Also, anyone got any first hand experience of the LDR jobbies with a SS muscle amp to share? Rest of my system should be in sig.

sure - i've own both myth LDR and first watt F3 clone and it's a combo capable of outstanding results, transparency & clarity married with a class A warmth (though these days i use my 10Y SET amp more often). pretty much the same with restored krell KSA-50 - myth is completely insensitive to any source or power amp imp issues due to buffers. myth/F3 combo was 3 ohm into 10k, and furthermore my DAC (i used at time) had 2.7k so actually it was 2.7k -> 3ohm ->10k.

so, be sure to explore the appropriate buffers if you go DIY route.

[rahman]

Ta muchly for the input everyone. Really appreciated. And particularly nice to have input from the designer too! Really nice work btw.

Have been digging around more (though the DIYaudio.forums on this topic are waaay over my head) and I think the way forward will be a combination of a pass b1 buffer with a LDR (probably the EVA at present as a multiple input version should be available soon - and it has a remote). It would be nice to have a one box solution - just wish I had the DIY skills to do it.

Gordan: I've noticed that you've mentioned the myth in a couple of places. Any details of its release date? And are they're still going with jfet buffers or have you convinced them to go valves?

Cheers
Adrian