Setup Sound Skulptor MP573

[slipperyflax]

First post and a newbie in the DIY-world. Like it so far though Anyhow, put together a MP 573 as a first build (have soldering experience since before though) and when it came to the setup I got these results:

Setup step:

1. Short circuit check: No problem, it was value greater than 1k ohm. However, it kept rising steadily, and didn’t really set at any value. Is this normal?
2. -
3. General power check: I have a Fredenstein Bento 6, The DMM read 25.2V both with the output transformer connected and without. Bad sign I guess?
4. Bias adjust: I had no problem here.
5. Sound check: I had no problem here really. However, between gain steps 20-30, 30-40, 40-50 and 50-60 there’s a popping sound.
6. DI Check: No problem.

Also, when having a guitar (humbucker) plugged in the DI, I feel that it’s kind of hard to get the unit to "distort/clip" before my interface does (Saffire Pro 40, with input pad engaged). When I have the MP573 set to LINE, I can have the GAIN knob all the way up but and the trim-knob all the way down, but the LED won’t turn red. I can make the LED turn red at other settings, and with a condenser/dynamic microphone, but I found it kind of baffling. I'm not necessarily implying that anything is wrong with the lack of  distortion/clipping at LINE input, just want to confirm this is the way it should be. My main concern is that the voltage didn't drop with the output transformer plugged in. I do realize that when designing a micpreamp like this back in the day you'd like to avoid distortion/clipping  :

I’ve tried to check solder joints but I can’t find anything that seems wrong. Nice and shiny joints but I guess I have a fault somewhere. What do you guys think? It doesn't seem to be anything that gets particular hot either. Could there be faulty resistors somewhere? or is everything fine?

EDIT
Schematics: http://www.soundskulptor.com/docs/mp573-schematic-02.pdf
Setup guide: http://www.soundskulptor.com/docs/mp573-setup-guide-02.pdf

 

[PRR]

Please *link* to product descriptions. Searching "MP 573" brings up a lot of pills. The way you slipped from Bento to DMM, I thought your DMM was a Bento. Nobody can remember all the million products out there.

"Short circuit check" could be many things. If you mean you measured across the supply rails, you should first read low because the rail caps are discharged, then come up as they charge, and the final reading is uncertain until/unless you know the circuit internals.

I have no idea what the 25.2V means. At power rails? At Bento? At transformer?

You would NOT switch gain mid-take. Some popping may be acceptable. Small popping may be unavoidable in high-gain circuits. Not going to hunt-down the MP's schematic and look.

No, this High Quality preamp will NOT "distort" before the next stage's input is screaming in pain. +27dBu is BIG signal, more than many interfaces can take. You want to flavor a guitar, use a much cheaper preamp and a pad between it and the interface.

 

[slipperyflax]

Please *link* to product descriptions. Searching "MP 573" brings up a lot of pills. The way you slipped from Bento to DMM, I thought your DMM was a Bento. Nobody can remember all the million products out there.

"Short circuit check" could be many things. If you mean you measured across the supply rails, you should first read low because the rail caps are discharged, then come up as they charge, and the final reading is uncertain until/unless you know the circuit internals.

I have no idea what the 25.2V means. At power rails? At Bento? At transformer?

You would NOT switch gain mid-take. Some popping may be acceptable. Small popping may be unavoidable in high-gain circuits. Not going to hunt-down the MP's schematic and look.

No, this High Quality preamp will NOT "distort" before the next stage's input is screaming in pain. +27dBu is BIG signal, more than many interfaces can take. You want to flavor a guitar, use a much cheaper preamp and a pad between it and the interface.

Well my apologies. I simply copy pasted the email I sent to JPK at Soundskulptor I think the easiest way to explain everything is with a link to the setup-guide: http://www.soundskulptor.com/docs/mp573-setup-guide-02.pdf

My main intent is not to use this to flavour a guitar-tone, but that was one of the tests in the setup guide. One has to try out the DI-input, right? I just want to confirm that everything seems to be alright. Since it is my first build and I'm not all that educated within electronics I rather be safe than sorry. I built this to learn and because it's fun to expand my "skills".

A link to schematics, if someone would like to have a look: http://www.soundskulptor.com/docs/mp573-schematic-02.pdf

Thanks.

 

[PRR]

> General power check: I have a Fredenstein Bento 6, The DMM read 25.2V both with the output transformer connected and without. Bad sign I guess?

""Plug in power. You should read about 24.2 Volts. This value may vary, depending on the lunchbox PSU. Plug off power. Insert the output transformer connector and make the same check. The voltage should be a little lower, around 24.0 Volts.""

As it says, "This value may vary". 25.2 is same-as 24.2 for our purposes. I'm not sure why the second sub-step assumes your power supply will droop when the added load of the output section (transistor and transformer) is added-- many power supplies will not sag enough to notice (and they should say so).

A further step is to smoke-test for a minute, then see if the big heat-sunk transistor gets warm/hot. If so, that is normal, and proves the output stage is doing a right thing (sucking DC power it can apply toward output audio power).

However, since in Bias Adjust you had "no problem here", and it does drive your interface, that transistor + transformer MUST be working. Otherwise you'd get no bias and nothing out.

> hard to get the unit to "distort/clip" before my interface

""DI check ... ... You should hear your instrument when playing.""

They only want "hear", no mention of distortability.

As I see it, your one true 'problem' is clicking on gain changes. That's 'cosmetic', it doesn't spoil the music. Back when BBC/ITC were buying studios-full of preamps like this, someone surely strived to minimize the clicks. But there is a lot going on there. Sometimes such click-reduction requires custom-cut switch contacts so make/break happens in a certain order. In the 1970s there were small switch-makers who would do such work. Today all switches are made in vast Asian factories and they don't like small runs of custom contacts.

 

[slipperyflax]

That sounds promising! Maybe I'll test and run it for a while and see what happens (other than great music, hopefully  )

Thank you for your time!

 

[JPK]

This is a copy of an email I sent to you 2 days ago. But all my emails to you are rejected with a 'Mail Delivery System' error!
Many thanks to PRR for trying to sort out the problems.

1. Short circuit check: No problem, it was value greater than 1k ohm. However, it kept rising steadily, and didn’t really set at any value. Is this normal?
Yes normal, there are capacitors that are charging from the meter voltage.
2. -
3. General power check: I have a Fredenstein Bento 6, The DMM read 25.2V both with the output transformer connected and without. Bad sign I guess?
Not necessarily. If you can do the following bias adjust then all is OK.
4. Bias adjust: I had no problem here.
5. Sound check: I had no problem here really. However, between gain steps 20-30, 30-40, 40-50 and 50-60 there’s a popping sound.
Check you do not have a reversed electrolytic or tantalum cap.
6. DI Check: No problem.

Also, when having a guitar (humbucker) plugged in the DI, I feel that it’s kind of hard to get the unit to distort before my interface does (Safire Pro 40, with input pad engaged). When I have the MP573 set to LINE, I can have the GAIN knob all the way up but the LED won’t turn red.
This is normal. In line mode the input signal is attenuated by 26dB.
I can make the LED turn red at other settings, but I found it kind of baffling. I’ve tried to check solder joints but I can’t find anything that seems wrong. Nice and shiny joints but I guess I have a fault somewhere. What do you think?
You could make the value of R21 smaller or even short it with a wire to increase the maximum attenuation of the trim pot but this would break the -10/+10dB law.

 

[slipperyflax]

Fantastic JPK! Thank you. Next month I'll order another kit from you  8)

 

[slipperyflax]

I actually had a reversed tantalum cap and now the popping went away So now everything is tested, completed and sounds fantastic!

 

[PRR]

Thanks, JPK.

 

[Gene Pink]

I actually had a reversed tantalum cap and now the popping went away

Usually reversed tantalums only pop once, and they are loud, spewing chunks everywhere. In power supply circuits.

I wouldn't trust a tant that has seen any real reverse voltage, in anything critical. They are not forgiving to this.

Did you replace it?

Gene

 

[slipperyflax]

Usually reversed tantalums only pop once, and they are loud, spewing chunks everywhere. In power supply circuits.

I wouldn't trust a tant that has seen any real reverse voltage, in anything critical. They are not forgiving to this.

Did you replace it?

Gene

:-[ No I did not! I have no replacements. Hmm.. I did the setup over again and it seemed to be working fine but maybe I should replace it to be sure! The damnest thing is that we don't have any suppliers for this in my little town (= I can't fix it before the weekend).

 

[JPK]

If the cap didn't break (which is the case because otherwise the pre wouldn't work) then I think you are safe.