[CurtZHP]

My most recent mic preamp project, "THAT Thing," has been working great for me.  Recently, though, while experimenting with some mic setups, I came upon a rather odd circumstance when using it with another gadget in my studio.  I've attached the schematic for reference.

As I said, under normal circumstances it performs satisfactorily.

When I plug in this gadget...
https://www.radioworld.com/tech-and-gear/simply-sound-ss1-gives-mics-a-pick-me-up

....suddenly, all the low end disappears, and what's left sounds really boxy and harsh.

A little background on this SS-1 gadget...
I received it a couple years ago when I did a review of it for Radio World.  The makers let me keep it.  It works great on every other preamp I've ever used it with.  The company, Simply Sound, has apparently dropped off the face of the earth, so I have no way of asking them; and I have no schematic for it.

I've tried it with transformer-coupled preamps, capacitor-coupled preamps, and whatever my Audient iD22 uses.  Works fine.  But, as soon as I plug it into THAT Thing, it sounds not much better than a telephone line.

Obviously, there's some weird interaction between the two that doesn't seem to happen elsewhere. 

Thoughts?

P.S. the mic I'm using is an MXL R40 ribbon mic.  I also tried an Electro-Voice RE320.

[CurtZHP]

Might have found the issue.  Or at least another clue.  It seems things improve considerably when I flip the polarity switch on the preamp to the "inverted" position.

[squarewave]

Just to be clear, you're putting the SS-1 between the mic and the THAT pre? With the polarity switch on the output, it's not clear why that would interact with something connected to the input.

What are the DC voltages on the output pins 2 and 3. Just wondering what kind of offset's you're getting there.

Is it possible you have your HPF in? I'm sure you're aware that the corner frequency is gain dependent. For the highest gain position, the cutoff is going to be over 700 Hz. I have a THAT pre with a HPF just (two frequencies actually) like that but I only went up to maybe 250Hz or so in the highest gain position. I'm assuming you did that so that you could make the 1u a film cap. I like it but you have to know how it works.

[CurtZHP]

Just to be clear, you're putting the SS-1 between the mic and the THAT pre? With the polarity switch on the output, it's not clear why that would interact with something connected to the input.

Yes, the SS-1 is between the mic and the preamp.  It requires phantom power, so that's really the only way to use it.  I can't see how the polarity switch would matter in this case, unless this particular combination of gear somehow screws with the line input on the Audient.  I didn't have anything else handy to plug into for comparison.  This is made further odd by the fact that none of my other preamps, connected to the Audient the same way, exhibit this behavior when using the SS-1.

What are the DC voltages on the output pins 2 and 3. Just wondering what kind of offset's you're getting there.

52VDC on each pin with nothing plugged in.

Is it possible you have your HPF in? I'm sure you're aware that the corner frequency is gain dependent. For the highest gain position, the cutoff is going to be over 700 Hz. I have a THAT pre with a HPF just (two frequencies actually) like that but I only went up to maybe 250Hz or so in the highest gain position. I'm assuming you did that so that you could make the 1u a film cap. I like it but you have to know how it works.

That would not entirely surprise me as the culprit, since the HPF is gain dependent, and we're adding about 25dB right out of the starting gate.  I did, however, not have the filter engaged.  I confirmed this by actually switching it on, and immediately heard the difference.

As for my choice of cap, it was combination of what I had around and just picking one by ear.  The math, as you've seen, says that's excessive for a HPF.  I've been kicking around resizing it, but just haven't had the time to do it.  This may just be the excuse I needed.

I suppose I could find a larger audio grade electrolytic of the same series as the big 3300uF.  I haven't run the numbers yet, but is it possible that the extra 25dB of gain is sufficient to cause a roll-off even through that large a capacitor?  Seems unlikely.

One of these days, I think I'll crack that SS-1 open and see what's going on in there.  They claimed it does more than just boost levels, so who knows how many other "undocumented" filters are active here.

[abbey road d enfer]

Thoughts?

Have you noticed, with the setups that work, that you need to crank up the gain?
What happens if you increase C11/12, to at least 100uF?
I suspect something weird with the input impedance.

[CurtZHP]

Have you noticed, with the setups that work, that you need to crank up the gain?

I haven't had to crank the gain on the others.  At least no more than on this one.

What happens if you increase C11/12, to at least 100uF?

Haven't tried that. 

[squarewave]

Maybe you need something that sets the impedance of the THAT1512 input. Right now you have the transformer which is just going to reflect what it sees upstream which is the phantom power resistors which are pretty high Z and the SS-1 device. So maybe there's something weird going on there. Obviously with that transformer it's load needs to be ~50K. So perhaps try putting a 47K or 100K across the secondary if only momentarily to test. It's also not crystal clear to me that the THAT1512 is supposed to work with such a high Z. I don't see why it wouldn't but all of the examples in the datasheet equate to quite a bit lower.

It's also not crystal clear to me that you need or want to directly ground the negative input of the THAT1512. You could have two 100K to ground instead.

But I must admit I'm at the limit of my understanding of such things.

[CurtZHP]

But I must admit I'm at the limit of my understanding of such things.

Which still puts you way ahead of me.

[abbey road d enfer]

OK, I didn't catch that this SS1 thing is connected between mic and preamp.
So, this SS1 probably does not like the low inductance of the OEP xfmr.
With a 200r source, the LF -3dB point is 60Hz.
I would think the SS1 has a significantly higher output Z.
You need to evaluate the output Z of the SS1.
Drive it with a steady signal ; make sure the source impedance is about 150-250 ohms (put a resistor in series if your source has a lower impedance). Measure the output without any significant load (if you use your DAW, the typical 10-20k input Z of a souncard is close enough to no load). then load the output with a resistor, adjust the value until the level drops by 6dB (half-voltage), then the value of this resistor is roughly the value of the output Z of the SS1.
I guess the actual output Z of the SS1 is in the order of magnitude of 1k.
This OEP xfmr is a lousy piece. The primary inductance (0.5H for two primaries in series) is just about 1/10th of  a good xfmr.

[CurtZHP]

OK, I didn't catch that this SS1 thing is connected between mic and preamp.
So, this SS1 probably does not like the low inductance of the OEP xfmr.
With a 200r source, the LF -3dB point is 60Hz.
I would think the SS1 has a significantly higher output Z.
You need to evaluate the output Z of the SS1.
Drive it with a steady signal ; make sure the source impedance is about 150-250 ohms (put a resistor in series if your source has a lower impedance). Measure the output without any significant load (if you use your DAW, the typical 10-20k input Z of a souncard is close enough to no load). then load the output with a resistor, adjust the value until the level drops by 6dB (half-voltage), then the value of this resistor is roughly the value of the output Z of the SS1.
I guess the actual output Z of the SS1 is in the order of magnitude of 1k.
This OEP xfmr is a lousy piece. The primary inductance (0.5H for two primaries in series) is just about 1/10th of  a good xfmr.

My G9 clone uses the same input transformers, and that gets along just fine with the SS-1.  Then again, that's a properly designed tube circuit.

[abbey road d enfer]

Might have found the issue.  Or at least another clue.  It seems things improve considerably when I flip the polarity switch on the preamp to the "inverted" position.

The polarity switch being at the output of the preamp, it suggests some weird interaction between it and the souncard. When the preamp is used without the SS1, does the polarity switch acts funny?

[squarewave]

I haven't run the numbers yet, but is it possible that the extra 25dB of gain is sufficient to cause a roll-off even through that large a capacitor?

Note that the gain of the SS-1 has no impact on the corner frequency. The corner frequency is defined by RG and the cap. So in the highest gain position RG is 220 which with 1u is a corner of 723Hz. In the second highest position the corner becomes 153Hz. In the lowest gain setting the corner is 1Hz.

I personally really like the idea of making the gain network cap smaller because you're not amplifying the lows in the first place which is great for utilizing headroom of the amp. But for a wide range of gain, it actually doesn't work. In my pre, I think I did like 40dB - 65dB or so and with two switchable caps. With your setup, it actually only works in basically the second highest gain position. You could tweak your resistors to change the gain range and adjust the cap to make it more useful. And you could swap the toggle with an ON-ON-ON toggle to get two low-cut settings.

You're also leaving out a lot of gain. Instead of 220, the smallest RG could be more like 22 or even lower. That would give you the right gain for a ribbon mic. Then pick caps that give you reasonable corner frequencies in the highest gain settings.

Also, note that you could put the 150K between the cap and the switch common and then pick other resistors that, when in parallel give you the desired gain so that the switch is never left open which could cause some popping when switching gain.

[CurtZHP]

Note that the gain of the SS-1 has no impact on the corner frequency. The corner frequency is defined by RG and the cap. So in the highest gain position RG is 220 which with 1u is a corner of 723Hz. In the second highest position the corner becomes 153Hz. In the lowest gain setting the corner is 1Hz.

I personally really like the idea of making the gain network cap smaller because you're not amplifying the lows in the first place which is great for utilizing headroom of the amp. But for a wide range of gain, it actually doesn't work. In my pre, I think I did like 40dB - 65dB or so and with two switchable caps. With your setup, it actually only works in basically the second highest gain position. You could tweak your resistors to change the gain range and adjust the cap to make it more useful. And you could swap the toggle with an ON-ON-ON toggle to get two low-cut settings.

You're also leaving out a lot of gain. Instead of 220, the smallest RG could be more like 22 or even lower. That would give you the right gain for a ribbon mic. Then pick caps that give you reasonable corner frequencies in the highest gain settings.

Also, note that you could put the 150K between the cap and the switch common and then pick other resistors that, when in parallel give you the desired gain so that the switch is never left open which could cause some popping when switching gain.

I've actually been toying with two ideas...
1.  As you suggest, redo the gain and HPF setup to make both more effective.
2.  Do away entirely with the HPF and the polarity switch.

[CurtZHP]

The polarity switch being at the output of the preamp, it suggests some weird interaction between it and the souncard. When the preamp is used without the SS1, does the polarity switch acts funny?

Couldn't recall, so I tested this again.  Plugged a mic directly into the preamp -- no SS-1.  Flipping the polarity switch does slightly change the response.  It seems to get slightly louder.

I'll borrow a small mixer from work to connect this to, to see if it does the same thing.

[abbey road d enfer]

Couldn't recall, so I tested this again.  Plugged a mic directly into the preamp -- no SS-1.  Flipping the polarity switch does slightly change the response.  It seems to get slightly louder.

how did you check? Headphones and your own voice?

[CurtZHP]

how did you check? Headphones and your own voice?

It's what was handy.  Switched it mid-sentence and could definitely hear a difference.
Bringing another mixer home to try an A/B comparison.  Will likely recruit my son to do the talking.

[CurtZHP]

Tried the polarity switch test again, this time with a different device downstream of the preamp, and with another person talking into the mic.  Switching causes a slight tonal difference, especially at low frequencies, but the difference is slight enough that I would have a hard time calling it a problem.  It was likely exaggerated by my hearing my own voice in both the headphones and my own thick skull.  (Can one's head produce phase shifts or cancellations at low frequencies?)
And this is a feature I almost never use anyway.

In this case, it was just a rabbit trail, distracting me from the original problem outlined in this thread.

[CurtZHP]

Note that the gain of the SS-1 has no impact on the corner frequency. The corner frequency is defined by RG and the cap. So in the highest gain position RG is 220 which with 1u is a corner of 723Hz. In the second highest position the corner becomes 153Hz. In the lowest gain setting the corner is 1Hz.

I personally really like the idea of making the gain network cap smaller because you're not amplifying the lows in the first place which is great for utilizing headroom of the amp. But for a wide range of gain, it actually doesn't work. In my pre, I think I did like 40dB - 65dB or so and with two switchable caps. With your setup, it actually only works in basically the second highest gain position. You could tweak your resistors to change the gain range and adjust the cap to make it more useful. And you could swap the toggle with an ON-ON-ON toggle to get two low-cut settings.

You're also leaving out a lot of gain. Instead of 220, the smallest RG could be more like 22 or even lower. That would give you the right gain for a ribbon mic. Then pick caps that give you reasonable corner frequencies in the highest gain settings.

Also, note that you could put the 150K between the cap and the switch common and then pick other resistors that, when in parallel give you the desired gain so that the switch is never left open which could cause some popping when switching gain.

Thought this over and decided to make some calculations last night.  As you suggest, my present gain range of 19dB to 50dB is kind of useless for most applications.  Reran the numbers based on a range from 40 to 60, like you did with yours.  Allowed for the gain of the input transformer and the little bit of gain from the line driver and came up with a new series of RG resistor values.

Ran the numbers for the HPF again, based on the new RG values.  Obviously calls for a much larger HPF cap, but the corner frequency at each switch position is much more practical.  As you said, what I have now only gives me a usable HPF at one or two positions.  What's the point, at that point?

Scrounged enough parts to give this a whirl as soon as I have time to put it on the bench.

Thanks!

[squarewave]

As you said, what I have now only gives me a usable HPF at one or two positions.  What's the point, at that point?

The point is that it's a very efficient, easy and cost effective way to attenuate low frequencies to increase headroom when you need it which is at high gain.

Imagine a signal that has a 100Hz component and a 1kHz component. The waveform would be the 1kHz superimposed over the 100Hz. When the 100Hz crests, the 1kHz is what will clip first because it's riding on top of the 100Hz. If you attenuate the 100Hz, the 1kHz is no longer riding on top of the 100Hz and so the chances of clipping decrease significantly.

A conventional mic pre applies low-cut after amplification and therefore cannot achieve the same benefit of increasing headroom by attenuating low frequencies. You only get that in these CFIA style amps that have the gain control network integrated into the input.

Meaning it's ok that the low cut only works at high gain. It would be great if it worked equally well at all gain settings but it is still useful.

In practice, I tend to apply a lot of gain at the mic pre to maximize noise performance of downstream outboard gear. Then I literally just move the mic closer or further away to get a good level. Otherwise, if the mic is really close, I don't think it sounds as good. Of course this strategy only works in a quiet room.

[abbey road d enfer]

It's what was handy.  Switched it mid-sentence and could definitely hear a difference.

But it's wrong! Why? Because what you hear is the sum of the sound coming from the headphones and the sound coming from bone conduction. The two combine differently when polarity is switched.
I've seen too many people evaluating microphones in shops that way.

Will likely recruit my son to do the talking.

That is wise.

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