dmp
Well-known member
The top opamp is inverting while the bottom opamp is non-inverting. The gain is different, Rf/Rin vs 1+Rf/Rin
THAT1512 / Transformer 'blind test' mic preamp
- Thread starter jamesk
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The top opamp is inverting while the bottom opamp is non-inverting. The gain is different, Rf/Rin vs 1+Rf/Rin
amplexus
Well-known member
I’ve used both the DRV and THAT chips with no output coupling caps with no issues. Just make sure you have the caps to the SENSE lines and you should be golden.
dmp
Well-known member
Then why do the top and bottom have different pk-pk magnitudes?
dmp
Well-known member
It's in the spice simulation results
Wavelength
Well-known member
I have used both the DRV and the That parts. I would say hands down go That for the output. I direct coupled both but had issues with the DRV in protos so I talked the company I was designing for and the final design went with the That. I use the 1626 in a couple of 500 series modules I designed. The parts are great, another real nice sounding part is the That 1200 series for line inputs, stellar sounding. Used a set on an Audio Preamp board I am designing now.
soulsonic
Well-known member
Yes, I believe so. I made this same mistake on mine...
jamesk
Member
This is interesting. It's exactly the same circuit as in Self's book. I've just been having a look at the schematics for the desk and it's full of circuits from that book. I know he worked at Soundcraft but did he work at DDA when they made that, I wonder.
What's very interesting is that it says to calibrate the balance at 1kHz, which implies that the balance does vary with frequency, as I'm observing.
Hmm, that's annoying to hear that about the DRV as it's much cheaper than the THAT. Maybe I'll end up going with the quasi-balanced circuit.
Thanks, I'll do that!
I just want to check the overall grounding arrangement. I have the power supply in a separate steel enclosure. This will be connected to the preamp enclosure by a 5-pin XLR lead: +/-17V, +48V, signal ground and chassis ground. At the preamp end, signal and chassis ground will be connected to the two separate wires. These will then be joined at the power supply end to a star ground point on the enclosure, where the 0V from the power supply board also connects. Does this arrangement sound sensible or should I change something? I have also got separate ground connections on the PCBs for signal ground and the power ground from the decoupling electrolytics on the power input. Should this power ground be connected to the signal or chassis ground?
Thanks,
James
DaveDC
Member
As Ian correctly pointed out, the voltage balance of the two output signal legs is entirely irrelevant when driving a balanced input, since that input is only sensitive to the *difference* between the two signals. There is absolutely no need to tweak them for balance. The output impedance of each leg should indeed be matched so as to not degrade the common-mode rejection ratio (CMRR) of the balanced receiver downstream. But this only matters for the traditional “diff amp” style of balanced receiver, the one with two input resistors and a single opamp. Any imbalance in the impedances of the driving signal legs (which are in series with these input resistors) equates to an imbalance of the diff amp’s resistor “bridge,” which in turn reduces its CMRR. This is why the “impedance balanced” output stage works so well: it does not degrade the CMRR of the following stage. However as Bill Whitlock has been telling us for years, an input stage with high common-mode impedance like a transformer or an “instrumentation” style diff amp (INA163 type, with two unity-gain buffers in front of a regular diff amp) is not degraded by mismatched source impedances.
The true purpose of these cross-coupled output stages is to emulate the action of a transformer when the balanced output signal is converted to unbalanced by grounding the negative leg. Doing this with a transformer doesn’t change the balanced signal level, but rather just causes the ungrounded leg to double in voltage. These cross-coupled opamp circuits do the same thing. One caveat is that power supply voltage and signal level choices need guarantee that there will enough output voltage swing in the opamp to not clip when the opposite leg is grounded.
There is a pretty big difference when designing balanced drivers for stand-alone gear compared to internal signals within a console. For stand-alone gear, you have no knowledge of how or to what the user will connect your outputs. So the driver has to handle all kinds of contingencies, like short circuit and over voltage protection, etc. Within a console, it is possible to tailor the driver to match the receiver, since both are under the designer’s control. For a console, probably the best bet is to ditch the cross-coupling and go with one inverting and one non-inverting amp, both fed from the same preamp.
BTW, the imbalance at high frequencies you are seeing is probably due to capacitive imbalance from either the 22pF cap tolerances or your board layout. Or your opamps are too slow and are running out of gain at high frequencies.
The true purpose of these cross-coupled output stages is to emulate the action of a transformer when the balanced output signal is converted to unbalanced by grounding the negative leg. Doing this with a transformer doesn’t change the balanced signal level, but rather just causes the ungrounded leg to double in voltage. These cross-coupled opamp circuits do the same thing. One caveat is that power supply voltage and signal level choices need guarantee that there will enough output voltage swing in the opamp to not clip when the opposite leg is grounded.
There is a pretty big difference when designing balanced drivers for stand-alone gear compared to internal signals within a console. For stand-alone gear, you have no knowledge of how or to what the user will connect your outputs. So the driver has to handle all kinds of contingencies, like short circuit and over voltage protection, etc. Within a console, it is possible to tailor the driver to match the receiver, since both are under the designer’s control. For a console, probably the best bet is to ditch the cross-coupling and go with one inverting and one non-inverting amp, both fed from the same preamp.
BTW, the imbalance at high frequencies you are seeing is probably due to capacitive imbalance from either the 22pF cap tolerances or your board layout. Or your opamps are too slow and are running out of gain at high frequencies.
jamesk
Member
Thanks for your detailed response. I realise that the signal is differential but if the two signals are different amplitudes, the larger one will clip earlier, reducing the headroom compared to if they were matched. It's this that I'm concerned about. In a typical signal, though, the high frequency components will be much lower in amplitude anyway, I think I'm right ins saying, so maybe this isn't an issue.
Yes, this was why I went for this stage, and it is performing it's role of doubling the output voltage when connected to an unbalanced input correctly. Since the ultimate aim is to use it as an output stage on a desk, it will need to cope with balanced and unbalanced inputs, particularly on the inserts.
I think I've been a bit unclear about where the stage would feature. It's intended as an output stage on the desk to connect external equipment. The circuits and PCBs here are just to test the mic preamps, and this output stage, and aren't intended as part of a desk as they are. I haven't got a detailed plan yet but I think the mic preamp will just connect to the next stage single-ended. If there's any balanced signalling within the desk, I will follow your suggestion of using a straightforward balancing arrangement, given that connection to unbalanced equipment isn't an issue.
In terms of the frequency-dependent imbalance, I don't think it's down to the capacitor tolerances or layout as I'm getting the same behaviour in the SPICE simulation. I'll investigate your suggestion regarding op amp choice by trying a different model in LTSPICE. I'll see if a higher speed op amp displays the same behaviour.
Thanks again for your response,
James
DaveDC
Member
Re: clipping, for a console output you need to choose your signal levels and power rails to stay well below clipping for the worst-case unbalanced (one leg grounded) condition. In all other conditions, the levels will be lower and clipping will not be an issue.
Re: HF imbalance, I thought your board was differing from your simulation, but I reread the thread and realize I got that wrong. You could simulate to confirm that both legs are truly balanced at DC, but because your feedback is AC-coupled through the 47uF output caps, you will get erroneous sim results at DC. Try shorting the 47uF output caps in simulation to see if balance returns. You could also use a perfect opamp model (VCVS with infinite GBW) to see if the balance is flat with frequency. The clue here is that if the imbalance is frequency dependent, it has to be due to a capacitance, either caps you placed on the schematic or ones that are embedded in your component models.
Re: console output, a few things. One, realize that your balancing pot acts by loading one leg more than the other. That load balance will almost certainly go out the window once you connect this circuit to a real external load. So this balance problem is just academic anyway. Next, consider any fault conditions. You'll want to add output over-voltage protection for the case when someone accidentally plugs your desk output into some speaker terminals or something like that. It happens. You should also add ESD protection, which is over-voltage protection but much faster. EMI ferrite beads do a good job of that. Also check your board's short-circuit behavior by running it at full voltage into a dead short and verifying nothing overheats.
Re: mic input. I see you have the usual protection diodes D5 and D6 on the THAT152 version. Their role is to protect the opamp from the phantom blocking caps' discharge in the event of a shorted input. However if the short happens at the far end of an inductive mic cable, it is possible to get spikes of opposite polarity. So consider adding a second pair of diodes in the opposite polarity. Don't forget the ESD protection ferrites and possibly a common-mode choke for RFI protection. And you might want to consider adding a CMRR trim, although for really good CMRR you'll probably need to match the DC blocking caps. The transformer version doesn't need any of that stuff, which is why transformers are so wonderful.
Equipment inputs and outputs are surprisingly difficult to get right because of all the abuse they are subject to. DIY projects generally don't have to deal with this problem, but if you intend to ship thousands of units, it is something you want to pay attention to.
Re: HF imbalance, I thought your board was differing from your simulation, but I reread the thread and realize I got that wrong. You could simulate to confirm that both legs are truly balanced at DC, but because your feedback is AC-coupled through the 47uF output caps, you will get erroneous sim results at DC. Try shorting the 47uF output caps in simulation to see if balance returns. You could also use a perfect opamp model (VCVS with infinite GBW) to see if the balance is flat with frequency. The clue here is that if the imbalance is frequency dependent, it has to be due to a capacitance, either caps you placed on the schematic or ones that are embedded in your component models.
Re: console output, a few things. One, realize that your balancing pot acts by loading one leg more than the other. That load balance will almost certainly go out the window once you connect this circuit to a real external load. So this balance problem is just academic anyway. Next, consider any fault conditions. You'll want to add output over-voltage protection for the case when someone accidentally plugs your desk output into some speaker terminals or something like that. It happens. You should also add ESD protection, which is over-voltage protection but much faster. EMI ferrite beads do a good job of that. Also check your board's short-circuit behavior by running it at full voltage into a dead short and verifying nothing overheats.
Re: mic input. I see you have the usual protection diodes D5 and D6 on the THAT152 version. Their role is to protect the opamp from the phantom blocking caps' discharge in the event of a shorted input. However if the short happens at the far end of an inductive mic cable, it is possible to get spikes of opposite polarity. So consider adding a second pair of diodes in the opposite polarity. Don't forget the ESD protection ferrites and possibly a common-mode choke for RFI protection. And you might want to consider adding a CMRR trim, although for really good CMRR you'll probably need to match the DC blocking caps. The transformer version doesn't need any of that stuff, which is why transformers are so wonderful.
Equipment inputs and outputs are surprisingly difficult to get right because of all the abuse they are subject to. DIY projects generally don't have to deal with this problem, but if you intend to ship thousands of units, it is something you want to pay attention to.
