Muffsy Pre PP4 input capacitor question from a beginner

[Bingod]

Hello everyone. I hope you can offer some help to a beginner who has only a very limited understanding of electronics, but is hoping to learn. I am currently building a Muffsy PP4 phono pre - my first DIY project. Its going well and I'm almost done. This will replace the inboard phono stage on my Pro-Ject Debut USB which I intend to bypass with hardwired phono cables At least that is the plan.

I am looking for advice on creating the input capacitance of my system to best suit my Ortofon OM10 which requites 200-500pF, which is pretty broad. I know I can choose the capacitance of the phono cables but my bigger question is with the Muffsy itself. The input capacitance of the Muffsy is listed as "optional" and the only info I can garner from the maker (who is otherwise very helpful) is that I can add input capacitors to the PCB up to 200pF. There are two empty slots, which appear to be for film caps on the PCB designated for this. On the Muffsy website he suggests that most people will not need to add capacitors as their tone arm wires and cables should provide enough in themselves and so does not include any caps for this purpose. From what I have read people seem to aim for a high input capacitance for the OM10, between 350 - 400pF, which i don't think I can get close to with just wires and cables. What do you advise? Be gentle and thank you in advance for taking the time.

hopefully there is an image here of the pre-amp board

 

[triode17]

Welcome to the AK forum, the best around. I just went through the same thing. Recently bought a Project Carbon Espirit and an Orto. 2M Blue. Its recommended capacitive loading is 150-300 pF. My tone arm wires are only 20 pF, my phono cables are 60 pF, that's 80. So I simply added 120 to the input of my phono preamp and set it at 200 pF. i ran some frequency sweeps and didn't notice any peaks but the sweep record has a rather fast sweep and its hard to tell if there is a sharp peak. Who's got a better way to do this?

 

[Binkman]

Attached link is a bit of a read but a good one. Read through it and Felix' from Australia noted for his tests. He cites your cart with his TT sounded better (less brite) toward the 500pf set up. Note that any particular model or custom TT can have a different 'voice' cart combo.

http://forums.stevehoffman.tv/threads/ortofon-om10-question.343871/

 

[Bingod]

Thank you. 2 basic questions. Did you add a total of 80pF, i.e. 2 x 40pF caps - one to each channel, and how did you determine the voltage of the caps. The Muffsy uses a !6VAC wall wart; I was going to guess at 25V - is that safe enough?

 

[Binkman]

You might have missed my post while editing? But for power in supply requirements the less noise the better and 16vac is recommended.. why go higher vac?

 

[Bingod]

You might have missed my post while editing? But for power in supply requirements the less noise the better and 16vac is recommended.. why go higher vac?

Exactly, i was typing my post at the time. Thank you for the link, it confirms what I had heard about the Ortofon. It appears from the pcb that the maximum capacitance I can add per channel is 200pF. So with tonearm and phono cables it will be a challenge to get up to 400pF. I will need to choose some high capacitance cable for the phono cables. something around 50pF per foot.

Also I realize my question about voltage was not very clear. I had meant to ask about the voltage of the caps not the wall wart. Should i match the voltage of the input caps to the output voltage of the OM10 (4mV) or can I go higher for a safer ceiling? Im learning as I go here. Does the voltage matter? What would be a good cap for this purpose. Im not sure what to search for

Im planning to solder stackable headers on to the pcb and have the possibility of trying out a few different caps and see if can hear a difference.

Thanks again

 

[Binkman]

I had meant to ask about the voltage of the caps not the wall wart.

Admittedly I'm not familiar with said project board... must be some example of a schematic of parts listed for an example build?
Perhaps post a link on the TT forum? or ask to move the thread there.

 

[gadget73]

The small value caps on the input probably don't even exist at less than 50v. No worries about using them with a few mv for this purpose.

 

[absolon]

You can add whatever capacitance you choose. The 200pF is a general suggestion, not a limit.

 

[Binkman]

I'm kinda stuck on OP plan to add ac supply versus the wall rat. My link provided the audio side of caps in pf.. tested for the op's ball park. However if providing ac supply?

 

[Lavane]

Isn't there a power supply PCB available for the Muffsy? Also the back panel should be available.

 

[gadget73]

I'm kinda stuck on OP plan to add ac supply versus the wall rat. My link provided the audio side of caps in pf.. tested for the op's ball park. However if providing ac supply?

Board inputs make me suspect it wants a split rail DC supply, so unless its involving a pair of wall warts that happen to be well matched in output I don't believe either power supply choice is a good one. If you feed it AC instead of DC, the caps aren't going to matter, the op amp is probably going to fry first.

 

[Binkman]

Gadget... I'm simply stating on said quote is that I don't know what the OP intends to do. Use a wall rat(s) or a supply.. most of these kit boards have(had) optional ps plans or assembled to wire in.

OP hasn't responded so we're all just speculating. for all we know he may have shelved it.

 

[Bingod]

Thank you everybody - this is all very helpful. Everything is soldered and wired and as soon as I can make a decision about these caps the sooner I will close up the box and plug it in.

The small value caps on the input probably don't even exist at less than 50v. No worries about using them with a few mv for this purpose.

Thank you. Thats what I had hoped. I looked on Mouser and couldn't find anything remotely close. I saw some Cornell Dubilier 100V and 200pF mIca caps and Kemet 40VAC 220pF Polypropylene Caps. The mica caps don't specify VAC or VDC, they just give a voltage value. I hope these could work? though i haven't yet decided on the exact pF value.

You can add whatever capacitance you choose. The 200pF is a general suggestion, not a limit.

Thanks - that should make selecting the phono cable easier.

I'm kinda stuck on OP plan to add ac supply versus the wall rat. My link provided the audio side of caps in pf.. tested for the op's ball park. However if providing ac supply?

The 16VAC 500mA wall wart is specified on the Muffsy website. I bought it from Schitt, I believe its the same one they use for their Mani.

Isn't there a power supply PCB available for the Muffsy? Also the back panel should be available.

Yes. My kit came with the power supply and I bought the back panel as well. I was a little shocked by how small everything was - but it went together easily - the instructions and step by step photographs are on the website. I also plan to add an LED

 

[Bingod]

A few more solders to do. Ill drop in the op amps and set the gain and impedance when all is done. I circled the position for the "optional" input caps.

 

[Lavane]

Did you go with the optional op amps? I've been considering building one of these.

 

[gadget73]

The mica caps don't specify VAC or VDC, they just give a voltage value.

If it doesn't specify, its usually DC. They should be perfectly fine though.

 

[Bingod]

Did you go with the optional op amps? I've been considering building one of these.

When I placed my order, it was for the PP3 and I chose the upgrade op amps. In the time it took to process my order he had upgraded to the PP4, which i believe has the upgraded op amps as standard. The wording on the website is a little vague but it specifies "Upgrade op amps" as if they are included.
This is what you get.

• 1x Burr-Brown OPA2134PA + 1x LM4562

There is no option to select an op amp upgrade any more. There are some other design and layout changes.

 

[Bingod]

Just thought I'd update this. Here is the Muffsy with with 220pF film caps added to the input in headers. I bypassed the internal phono pre in my Pro-Ject Debut USB, (well I stripped it out) and hard-wired phono cables to the tonearm cables with 4' of Belden 1505F - about 80pF. With tonearm wires and connectors I'm at about 330-350pF - guessing that the connectors must add a little. So about midpoint in the Ortofon capacitance loading recommendation. I have swapped in a pair of 330pF caps (total about 450pF) but found no improvement to the sound - in fact for what ever reason (subjective, real or imagined) i preferred the sound with the 220pF. Im very happy with it (a slight buzz in the left channel notwithstanding) - perhaps I will continue to play with it at a later date.


220pF caps installed plus LED on front panel

 

[Lavane]

Maybe bend the red wire on the power switch away from the RCA Jack's next to it along with the red wire that's really close to that little blue film cap? Chasing hum problems is always fun.