Cdp

Sound Skulptor MP 566 behaviour
« on: March 15, 2021, 10:18:54 AM »
Hi everyone, I'm new here, I hope I'm in the right section.

I've just built 2 MP 566 mic preamp, nice kits, working great on the fist try.
The sound is fine but i'm asking myself some questions.

When you push the gain harder, like when going from Lo to Mid or High gain, it seem that there is something like a hipass hapening, so the low end disapears a little.

Is this behaviour normal? Is it due to the topology of the preamp, or is it the tube that is acting like that? Or maybe something else I don't think about.

I'm asking if there is a way to adress this and have it more linear even at high gain settings.

I haven't the knowledge to figure it out myself, I hope some of you can share some thought :-)

Thank guys!

Cdp
« Last Edit: March 18, 2021, 06:06:42 AM by Cdp »


JPK

Re: Souns Skulptor MP 566 behaviour
« Reply #1 on: March 17, 2021, 03:34:35 AM »
This doesn't sound right. The frequency response should be ruler flat up to 20kHz.
You must check the usual suspects:
  • Forgotten solder joint,
  • Bad solder joint (reflow suspicious joints),
  • Reversed components (electrolytics, diodes, transistors, IC's). In case of IC's, probably destroyed, better replace,
  • Flipped IC's or transistors,
  • Trim all the component leads and test pins as short as possible. Nothing must touch the chassis plate.

Cdp

Re: Souns Skulptor MP 566 behaviour
« Reply #2 on: March 17, 2021, 05:12:28 AM »
Thank You Jean Pierre for your answer.

I'm in the studio tomorrow, I will open the boxes and check again. As I already double check everything, I may post some photos in case you can see something I don't see.

I will do some measurement and post the results for you to see clearly what i'm talking about

Thank


abbey road d enfer

Re: Souns Skulptor MP 566 behaviour
« Reply #3 on: March 17, 2021, 05:29:39 AM »
When you push the gain harder, like when going from Lo to Mid or High gain, it seem that there is something like a hipass hapening, so the low end disapears a little.
How are you testing that? Beware that when switching, the preamp presents a different load to the incoming signal. With pad engaged the impedance is essentially resistive, but when disengaged, the impedance is inductive, which makes a high-pass filter. It shouldn't be noticeable, so it looks like something is wrong, like defective fransformer or wrong capacitor value. I would tend to exclude unproper design, since Sound Skulptor is a knowledgeable company.
Can you produce a graph of the frequency response. Most DAW's permit that. Audacity for example.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Cdp

Re: Souns Skulptor MP 566 behaviour
« Reply #4 on: March 17, 2021, 02:21:49 PM »
How are you testing that? Beware that when switching, the preamp presents a different load to the incoming signal. With pad engaged the impedance is essentially resistive, but when disengaged, the impedance is inductive, which makes a high-pass filter. It shouldn't be noticeable, so it looks like something is wrong, like defective fransformer or wrong capacitor value. I would tend to exclude unproper design, since Sound Skulptor is a knowledgeable company.
Can you produce a graph of the frequency response. Most DAW's permit that. Audacity for example.

Hi abbey road d'enfer, I first noticed it when comparing the sound with another tube pre, by ear. then I did some measurement with fuzzmeasure, I will do it again tomorrow morning and post the result here.

Of course I'm not questioning the quality of the design, I'm just wondering if I had made something wrong, or if it was just normal behaviour.  The thing you mentioned about the pad seems to be exactly what I felt.

Thank for your answer :-)

Cdp

Re: Souns Skulptor MP 566 behaviour
« Reply #5 on: March 18, 2021, 05:29:53 AM »
Hello, I did some measurement, here are the results in the attached photos. This is very zoomed in so be carefull !

What I did was putting the in and out pots to 12 o'Clock, just changing the switch gain position, and adjusting the level of the signal to have (approx) the same output level between measurement.

If that is wrong, just let me know and tell me how to do it right, I'll try to!!

If every thing seems ok, I'll be fine. Jean Pierre, if you tell me there is something wrong, I will open the pres and send some pictures.

I also have the same measurement from another tube pre, but it's not the same topology. I don't know if it's usefull or relevant to post that here, so I won't do it unless some of you think it could be usefull.

Thank

« Last Edit: March 18, 2021, 05:47:14 AM by Cdp »

abbey road d enfer

Re: Souns Skulptor MP 566 behaviour
« Reply #6 on: March 18, 2021, 02:23:13 PM »
Hello, I did some measurement, here are the results in the attached photos. This is very zoomed in so be carefull !
Again, my question is what is the impedance of the source?
Transformer response is very sensitive to the impedance that drives them.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Cdp

Re: Sound Skulptor MP 566 behaviour
« Reply #7 on: March 19, 2021, 03:58:23 AM »
Quote
Again, my question is what is the impedance of the source?

Sorry, I didn't understand what you were asking. I'm using a soundcard for testing. It's an Apollo UAD, old firewire model, I can't find any info about its output impedance on the net... somewhere it say 300ohms, somewhere 600ohms...

abbey road d enfer

Re: Sound Skulptor MP 566 behaviour
« Reply #8 on: March 19, 2021, 06:41:49 AM »
Sorry, I didn't understand what you were asking. I'm using a soundcard for testing. It's an Apollo UAD, old firewire model, I can't find any info about its output impedance on the net... somewhere it say 300ohms, somewhere 600ohms...
Not an ideal source for testing mic inputs with transformer.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Cdp

Re: Sound Skulptor MP 566 behaviour
« Reply #9 on: March 19, 2021, 07:11:56 AM »
I know it's not ideal, but have nothing else for the moment.
I'll try to find a signal generator to do it properly.

Does it mean that the measures I've made are absolute crap and don't reflect the real behaviour of the pre?
As I said, I've made the same measurement on another tube pre and it appears very very flat

I'm new in all of that and try to learn, but it's a long path!

Thank for your time and answer
« Last Edit: March 19, 2021, 08:59:51 AM by Cdp »


abbey road d enfer

Re: Sound Skulptor MP 566 behaviour
« Reply #10 on: March 19, 2021, 12:59:51 PM »
I know it's not ideal, but have nothing else for the moment.
I'll try to find a signal generator to do it properly.

Does it mean that the measures I've made are absolute crap and don't reflect the real behaviour of the pre?
Not crap, but it makes me suspect the input transformer is low-ish in inductance.

You can make the measurement with a pad between the DAW and the mic pre input.
This results in a source Z of about 150 ohms.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Cdp

Re: Sound Skulptor MP 566 behaviour
« Reply #11 on: March 21, 2021, 06:17:40 PM »
Thank abbey road d'enfer !!

I'm not in the studio for the next two weeks, I'll try your pad when I'll be back


 

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