Feeler: Classic Solo

[ruffrecords]

When I use phantom power, I get some loud white noise on my condenser for about 3 seconds. It makes the sound again when I turn phantom off. It doesn't do that on other preamps. Is this reason for concern?

First, with the mic unplugged, check you get 48V between pin2 and pin1 and also between pin 3 and pin1. In this condition do you still get the white noise when the phantom switch is operated?
Cheers

Ian

 

[FarisElek]

I got all my biggest troubles straightened out. White noise ended up being the mic, it was only doing it sometime so it seemed like it wasn't doing it on other pres, but was.

Should CZ be shorted? I left it completely blank.
I'm getting some buzz, but I believe it's my power transformer. I twisted it 180 degrees and it helped some.

Ryan

 

[ruffrecords]

HI Ryan, yes Cz should be usually be shorted. Some input transformers need an RC network for the correct response but if you are using one of the recommended transformers from Jensen, Cinemag or Sowter then Cz should be shorted. I little bit of hum is almost inevitable with a mains transformer and a mic pre in the same box. I have even seen cases when hum has been induced into the output transformer. Try to keep the mains transformer to the rear of the enclosure and the input and output transformers near the front. As you have discovered, rotating then relative to each other is often helpful.

Cheers

Ian

 

[funkymonksf]

Would this work for input if I wired off board? It available with a 1:10 ratio.

Would the center taps needs to be connected? I suppose wiring it up would be interesting.

MXcs Series - 1/8W nickel core microphone level matching transformers with copper shield.

DesignThese audio matchers use high quality low noise components to deliver broadcast and recording quality sound. Primary (Input)Designed for a balanced or unbalanced microphone level signal. Secondary (Output)Designed for a balanced or unbalanced line level signal. Magnet WireAll units are...

edcorusa.com

 

[ruffrecords]

Would this work for input if I wired off board? It available with a 1:10 ratio.

Would the center taps needs to be connected? I suppose wiring it up would be interesting.

MXcs Series - 1/8W nickel core microphone level matching transformers with copper shield.

DesignThese audio matchers use high quality low noise components to deliver broadcast and recording quality sound. Primary (Input)Designed for a balanced or unbalanced microphone level signal. Secondary (Output)Designed for a balanced or unbalanced line level signal. Magnet WireAll units are...

edcorusa.com

Yes, it should work. Unfortunately it has no mu-metal shielding so you will need to keep it well away from mains transformers. The centre taps should be left unconnected (for some reason Edcor has centre taps on all its transformers). Let us know how it performs if you try it.

Cheers

Ian

 

[funkymonksf]

Thanks Ian,

Does the shield need to be connected to any point on the PCB?

 

[ruffrecords]

Thanks Ian,

Does the shield need to be connected to any point on the PCB?

Yes, it should be connected to 0V.

Cheers

Ian

 

[My3gger]

Would this work for input if I wired off board? It available with a 1:10 ratio.

Would the center taps needs to be connected? I suppose wiring it up would be interesting.

MXcs Series - 1/8W nickel core microphone level matching transformers with copper shield.

DesignThese audio matchers use high quality low noise components to deliver broadcast and recording quality sound. Primary (Input)Designed for a balanced or unbalanced microphone level signal. Secondary (Output)Designed for a balanced or unbalanced line level signal. Magnet WireAll units are...

edcorusa.com

I tried 1:5 without mu metal shield with psu trafo out of chasis, wired p2p in typical paranoid way (p2p makes me act this way for some reason I don't think doctor will understand trouble with doing fully diff. AM5116b p2p...
It worked with all sorts of unwanted interference, you will be better with OEP or others having proper mumetal shield for mic inputs. Another option is adding shield but cost would be close to what good, ready made mic ins go for.
Ian, i believe Edcor adds CT connections because this allows using some of them for push pull, changing ratios, using half of the winding and so on. Doing it cost them very little.

 

[funkymonksf]

So what's the deal with super small holes on C13 (early solo PCB, not the "lo-hum")?

Does anyone have an idea of how to thin out my capacitor's lead? Maybe you've got a better idea?

A couple more input transformer questions, if I was to go the VTX or OEP (1:645 ratio) or even EA2622 1:7 route, would I need to change any caps and/or resistors on the PCB? Would I lose gain? If so, how much?

I'm on the fence as to whether to just fork up the $65 a piece for the Cinemag CMMI-10APC or save a bit on the EA2622 for $45 and a lot more on the OEP/VTX $25.

If anyone has experience with those, what are your thoughts? Is it worth it to spend twice and go with the Cinemag? Or can recommend another input transformer option(s)? I'm also checking out the Crimson CT-110MP, as it is pinned out perfectly w/ 1:10 ratio.

 

[ruffrecords]

So what's the deal with super small holes on C13 (early solo PCB, not the "lo-hum")?

The last new version of the PCB was in 2016. I don't remember details of prior boards. But I just looked at the PCB design file for the original and the holes are 40mil or just over 1mm and all subsequent ones are the same. Plenty big enough unless you have chosen some unusual electrolytic capacitor. The recommended Farnell part has 0.5mm diameter leads.

A couple more input transformer questions, if I was to go the VTX or OEP (1:645 ratio) or even EA2622 1:7 route, would I need to change any caps and/or resistors on the PCB? Would I lose gain? If so, how much?

I'm on the fence as to whether to just fork up the $65 a piece for the Cinemag CMMI-10APC or save a bit on the EA2622 for $45 and a lot more on the OEP/VTX $25.

If anyone has experience with those, what are your thoughts? Is it worth it to spend twice and go with the Cinemag? Or can recommend another input transformer option(s)? I'm also checking out the Crimson CT-110MP, as it is pinned out perfectly w/ 1:10 ratio.

If you want to use an OEP mic transformer I would recommend the A187A15C but by the time they get shipped to your side of the pond you might as well pay the $65 for a proper Cinemag. I wish I could buy Cinemags for $65 landed in the UK. By the time they are shipped and dutied over hear they cost as much as a Sowter but they are my number one mic transformer choice (closely followed by Jensen). Anything cheaper than these will likely not have sufficient screening. This is the most sensitive lowest level part of the circuit and I think it is worth spending a few bucks more to here where it really counts.

Cheers

Ian

 

[funkymonksf]

It’s been awhile since I’ve been researching this project, so I’m sorry if this is redundant…

Is there a way to add a lo and hi pass filter to this circuit? If so, how?

 

[ruffrecords]

It’s been awhile since I’ve been researching this project, so I’m sorry if this is redundant…

Is there a way to add a lo and hi pass filter to this circuit? If so, how?

You can easily add a high pass filter. There is a basic design shown in the documentation. Go to the DIY page of my website:

Custom Tube Consoles - DIY

scroll down and click on the Classic Solo folder. In their you will find the build instructions which includes details of a HPF. In fact you can add pretty much any pof my passive EQ designs between the level pot and the input of the second amplifier. Be aware the overall gain will be reduced by the insertion loss of the EQ.

Cheers

Ian

 

[funkymonksf]

What reduction in gain is to be expected with using a 1:7 ratio input transformer compared to 1:10?

 

[ruffrecords]

What reduction in gain is to be expected with using a 1:7 ratio input transformer compared to 1:10?

In theory 1:10 gives you 20dB gain whereas 1:7 gives you nearly 17dB so the difference is 3dB. In practice they both give a little less but the difference is still close to 3dB.

Cheers

ian

 

[funkymonksf]

Thanks for all the helpful information and quick responses.

Question on the high pass filter, is there a way you can change the spst on-off-on toggle switch to a 3 position rotary switch? It’s mostly for my aesthetic preference and feel. What changes, if any, would I need to make to your hpf diagram?

 

[JMan]

Question on the high pass filter, is there a way you can change the spst on-off-on toggle switch to a 3 position rotary switch? It’s mostly for my aesthetic preference and feel. What changes, if any, would I need to make to your hpf diagram?

Ian will no doubt have the official answer, but if you’re looking to replace an spst toggle switch, you’d want a 2-position rotary, right?

Also, I am admittedly not familiar with Ian’s circuit, but something seems off with the description of an spst on-off-on switch…how is that even possible? Is this perhaps meant to be a non-shorting spDt? Or perhaps an sp3t?

 

[Walrus]

Look at the link Ian posted earlier, Classic Solo Low Hum build instructions.

 

[ruffrecords]

Ian will no doubt have the official answer, but if you’re looking to replace an spst toggle switch, you’d want a 2-position rotary, right?

Also, I am admittedly not familiar with Ian’s circuit, but something seems off with the description of an spst on-off-on switch…how is that even possible? Is this perhaps meant to be a non-shorting spDt? Or perhaps an sp3t?

SPDT switches with a centre OFF position are readily available. The filter works by having a CR circuit when in the off position. One of the on positions shorts the capacitor for a flat response and the other adds a cap in parallel to change the HPF frequency.

To use a three way rotary you pretty much wire it up exacltly the same way and just connect nothing to the middle position.

Cheers

Ian

 

[bigidibo]

Just finished my Classic Solo and I couldn't be happier with it. Phantom power is still "reserved for future expansion", but it sounds fantastic even with an SM57 or a line source going through it. And naturally, with a 12 position switch, I couldn't resist making it go to 11. Thanks so much for making this possible, Ian.

View attachment 93688

Where did you get the case? Very nice looking.

 

[JMan]

SPDT switches with a centre OFF position are readily available. The filter works by having a CR circuit when in the off position. One of the on positions shorts the capacitor for a flat response and the other adds a cap in parallel to change the HPF frequency.

To use a three way rotary you pretty much wire it up exacltly the same way and just connect nothing to the middle position.

Cheers

Ian

I should have looked at the docs. I was trying to answer funkymonksf’s question and became too fixated on the “spst” part of it, which I would have realized was just a mistake had I looked at the schematic.