Low output level of Randall speaker emulator circuit

[Ampere]

The amp is in for repair because of low out put on the XLR. Indeed it has low output, but it seems to me it's just designed that way. I measured the same voltages/signal levels as in the simulation within 5mV. The circuit consists of two voltage dividers that drop the signal level considerably and unity gain voltage followers with some tone shaping. The 10k:600 transformer further drops the level going out.
The amp works normal going to speaker. Measurements were made using true RMS meter and confirmed with a scope. The transformer tested good with 1k primary and 64 ohms DC resistance.

My question is, how is that supposed to be near any acceptable amount of line output.

Schematic:
https://el34world.com/Forum/index.ph...0;attach=68449

[Helmholtz]

What makes you think that the emulator circuit doesn't perform as it should?

What was connected to the XLR output?

The emulator input voltage can get much higher than what you used for simulation because the Q15 stage has lots of gain and can deliver up to maybe 20Vrms.

If the customer needs more output level you could increase R166 - but consider max. input level of the opamps.

[Leo_Gnardo]

My question is, how is that supposed to be near any acceptable amount of line output.

With all that padding, then a 10K:600 transformer following, it's intended to present a mic level signal, not a line level.

If you want to boost that level, you can try changing values in the first voltage divider made of 820K and 39K resistors. Either reduce the 820K or increase the 39K to your taste. Your schematic shows a generator output of 1.3 Vrms. If that's supposed to represent the speaker output of the amp, consider there's going to be approximately 20x more voltage driving the circuit when playing the amp loud, and you want to avoid overloading the op amps. From what I see, there's not a whole lot of wiggle room to boost the XLR output by reducing loss in the voltage divider and stay out of trouble when driving the amp hard. OTOH choosing a different transformer could get your customer where he wants to go if he needs a line level signal. That final op amp is driving a 1K load resistor. It could just as easily be driving the primary of a lower-ratio transformer, maybe even a 600:600, that would boost your XLR signal considerably.

[J M Fahey]

The amp is in for repair because of low out put on the XLR. Indeed it has low output, but it seems to me it's just designed that way. I measured the same voltages/signal levels as in the simulation within 5mV.

Which is ????????????
WHAT voltage re you measuring?
Where?

Remember to mesure it from XLR pin 3 to pin 2, that voltage is floating, no reference to ground.

My question is, how is that supposed to be near any acceptable amount of line output.

As mentioned above, that is microphone level.
Anything from 20mV to 100mV is fine.

And if customer is playing at bedroom level, ALL levels become low too.

[Ampere]

Sorry to take so long toget back, and thanks for your replies!

What makes you think that the emulator circuit doesn't perform as it should?

That's what I'm trying to figure out. I think it does, but the customer says it's gone weak. I'm suspecting he might have something on his mixer maybe that's causing the lower signal.
My question is about the design of the circuit in general, and if it is acceptable output level.

What was connected to the XLR output?

My customer said he was using it to feed a mixer input or was it an insert, I forgot.
I was testing it on mic preamp.

The emulator input voltage can get much higher than what you used for simulation because the Q15 stage has lots of gain and can deliver up to maybe 20Vrms.

The test was made with all the preamp knobs at noon. I measured to about 2Vrms, before significant breakup. The master was set to 0. Turning the master past a quarter turn would get close to needing earplugs hooked up to one 12" speaker.

If the customer needs more output level you could increase R166 - but consider max. input level of the opamps.

Thanks for the suggestion. This was my first thought also.

I returned the amp to the customer after rising the level a bit. I'm still waiting to hear back from him.

[Ampere]

With all that padding, then a 10K:600 transformer following, it's intended to present a mic level signal, not a line level.

If you want to boost that level, you can try changing values in the first voltage divider made of 820K and 39K resistors. Either reduce the 820K or increase the 39K to your taste. Your schematic shows a generator output of 1.3 Vrms. If that's supposed to represent the speaker output of the amp, consider there's going to be approximately 20x more voltage driving the circuit when playing the amp loud, and you want to avoid overloading the op amps. From what I see, there's not a whole lot of wiggle room to boost the XLR output by reducing loss in the voltage divider and stay out of trouble when driving the amp hard. OTOH choosing a different transformer could get your customer where he wants to go if he needs a line level signal. That final op amp is driving a 1K load resistor. It could just as easily be driving the primary of a lower-ratio transformer, maybe even a 600:600, that would boost your XLR signal considerably.

Thanks for the reply.
I also thought if id boost the signal by omitting C37 and lowering R67, or would this change the tone too much? I added the final 1K resistor to the simulation to approximate the transformer primary, although it's impedance will be higher with a 1k signal. It didn't make much of a difference one way or another.

[Helmholtz]

I also thought if id boost the signal by omitting C37 and lowering R67, or would this change the tone too much?

That would significantly change the filter characteristic of the emulator circuit, which has the purpose of emulating the frequency response of a speaker.

U6A is wired as an active filter.

[Ampere]

Which is ????????????
WHAT voltage re you measuring?
Where?

Remember to mesure it from XLR pin 3 to pin 2, that voltage is floating, no reference to ground.




As mentioned above, that is microphone level.
Anything from 20mV to 100mV is fine.

And if customer is playing at bedroom level, ALL levels become low too.

The voltages I'm referring to are shown in the picture I posted. There are five nodes with V(rms) readings.
I guess there was no problem in the first place. I had the amp come in for some repairs. And one problem that the customer claimed was that the output from the XLR out had gone weak. He said he was using it only to feed a mixer input and had the master at 0. He said he used to have his fader at 4 (i dont know what the scale is) but now needed to have it maxed out so he'd get too much noise. I said I found nothing wrong with the XLR out, but he brought it back insisting there's something wrong. So you are confirming I was right, but I just wanted a second opinion. I did incerease the level of the XLR, and am waiting to hear back from him.

[Leo_Gnardo]

Thanks for the reply.
I also thought if id boost the signal by omitting C37 and lowering R67, or would this change the tone too much? I added the final 1K resistor to the simulation to approximate the transformer primary, although it's impedance will be higher with a 1k signal. It didn't make much of a difference one way or another.

I s'pose reducing R67 would buy you some gain. But after reviewing the dialogue from days ago, Randall's owner claims the DI output dropped noticeably. And here we are trying some circuit changes, instead of finding & correcting what I assume is a sudden onset problem. But, as you mentioned, the problem might be outside the amp, in his mixer. From what you've told us, the DI out seems to be working OK. Especially if it can deliver 2V clean signal, should be enough for just about anything. It's a real head scratcher. I'm going to have to toss in the towel now, but I'll be watching to see if a solution is found.

[Ampere]

I s'pose reducing R67 would buy you some gain. But after reviewing the dialogue from days ago, Randall's owner claims the DI output dropped noticeably. And here we are trying some circuit changes, instead of finding & correcting what I assume is a sudden onset problem. But, as you mentioned, the problem might be outside the amp, in his mixer. From what you've told us, the DI out seems to be working OK. Especially if it can deliver 2V clean signal, should be enough for just about anything. It's a real head scratcher. I'm going to have to toss in the towel now, but I'll be watching to see if a solution is found.

You're absolutely right there. The amp came back, and this time I finally caught it. It turned out to be a bad solder joint on a connector between boards. I did suspect that connector before and even cleaned it. Worked fine after that, but i guess the cleaning made the cracked solder work momentarily so I missed it. So it was just some sloppiness on my part there, and then I got side tracked. Hope I learned something. Probably not.