VU BUFFER & PEAK LED KIT BUILD THREAD

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Joe Malone
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Joe Malone »

jrowell wrote:
Joe Malone wrote:
api wrote:Hi.

I just ordered a large quantity of the these pcb´s which are going into a vintage Neve that i built a custom VU bridge for.
There will be 16 small Simpson VU´s and 5 larger ones.

My question is if i can omit the parts on the pcb that relates to the peak LED?
I will most likely not use this feature and wanted to save a few bucks on parts and also save time when building them all.
Yes you can leave off D4 BAT85, C3 1uF, R9 1Meg, R10 47k, RV2 5k trimpot, RED LED.

So spare opamp half doesn't now oscillate solder opamp pin 4 & 5 together and 6 & 7 together.
Hi,

To avoid unused op amp section being driven to negative rail connect Pins 6 and 7. Pin 5 must go to 1/2V source for single ended PSU topology. Re:
https://www.maximintegrated.com/en/app- ... vp/id/1957

James Rowell
While I agree with the link in normal precision audio path setups. Not in this case as with TL072 in VU buffer sharing the 1/2v with another input just makes opamp lock up more common at high gain setting. If you want to do this you must down size the 2 x 47k half rail resistors down to 2 x 10k and increase the half rail cap to 100uF to stop large turn on thumps pinning the VU meter to Full Scale and not have lockup problems. This mod uses more current than driving the spare opamp to 0v.

Or easier just do as I said originally as the extra current is tiny in TL072 and noise floor change is minimal and not seen due to the diode bridge feeding the meter.
Joe :-)
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mrclunk
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by mrclunk »

If i want to sum a balanced stereo signal to mono for inputing to the one of these boards, what's the best way?
This would hang off the mix bus so wouldn't want it to affect signal.
Do i need to add a buffer?

Is the circuit attached overkill?
balmix.jpg
thanks
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Merlot
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Merlot »

Hey Joe,

I plan on building a tube mic preamp and would like to add a vintage Telefunken VU meter. I wanted to ask, what the best way would be to power the buffer from the power rails I have.
The power transformer has 250V 20mA, 6,3V 700mA and 6,3V 500mA.
I wanted to test AC heaters plus de-hummer pot first and if it's too noisy connect the two 6,3V lines and rectify it.
Can you tell me how much current the buffer without peak LED consumes?
Thought about using the B+ rail with a series resistor. But I'm not sure if the trafo has enough juice for that as the B+ needs about 8mA (x1,6 for DC rectification so around 13mA)...
If I use DC heaters I could just use that as it would have around 18VDC right after rectification. Or does the buffer rail have to be well filtered?

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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Joe Malone »

mrclunk wrote:If i want to sum a balanced stereo signal to mono for inputing to the one of these boards, what's the best way?
This would hang off the mix bus so wouldn't want it to affect signal.
Do i need to add a buffer?

Is the circuit attached overkill?balmix.jpg
thanks
That circuit will unbalance the balanced summing a bit so is not a good idea
Joe :-)
JLM Audio
Capturing Audio without Injury

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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Joe Malone »

Merlot wrote:Hey Joe,

I plan on building a tube mic preamp and would like to add a vintage Telefunken VU meter. I wanted to ask, what the best way would be to power the buffer from the power rails I have.
The power transformer has 250V 20mA, 6,3V 700mA and 6,3V 500mA.
I wanted to test AC heaters plus de-hummer pot first and if it's too noisy connect the two 6,3V lines and rectify it.
Can you tell me how much current the buffer without peak LED consumes?
Thought about using the B+ rail with a series resistor. But I'm not sure if the trafo has enough juice for that as the B+ needs about 8mA (x1,6 for DC rectification so around 13mA)...
If I use DC heaters I could just use that as it would have around 18VDC right after rectification. Or does the buffer rail have to be well filtered?
I need to see the exact power supply circuit to advice the best way to do this.
Joe :-)
JLM Audio
Capturing Audio without Injury

Merlot
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Merlot »

It will be this one but instead of the rectifier tube I will use diodes.
I plan on using AC heaters but when that's too noisy I'll go with CR filtered DC heater.
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Joe Malone »

Merlot wrote:It will be this one but instead of the rectifier tube I will use diodes.
I plan on using AC heaters but when that's too noisy I'll go with CR filtered DC heater.
Due to the 6.3vac being centre ground 0v it will not work to drive the buffer.

The CR filter DC heater will work if you ground 0v one side so you have at least 9vdc
Joe :-)
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Merlot
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Merlot »

Hey Joe,

thanks for the reply.
I did not mean to use the 6.3vac to drive the buffer. I thougth about using the B+ with a dropping resistor.
And as a last resort I could use DC heaters and then thought about using this for the buffer. Thats why I ask if the supply voltage has to be well filtered for the buffer.
Can you tell me, how much current the buffer needs without the LED?

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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

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Merlot wrote:Hey Joe,

thanks for the reply.
I did not mean to use the 6.3vac to drive the buffer. I thougth about using the B+ with a dropping resistor.
And as a last resort I could use DC heaters and then thought about using this for the buffer. Thats why I ask if the supply voltage has to be well filtered for the buffer.
Can you tell me, how much current the buffer needs without the LED?
Buffer needs 10 to 20mA depending on meter type it is driving so drop resistor would need to 10W type and be wasting 3 to 6w of heat so not a good idea but could be done with 33v 5w zener across the buffer power input.
Joe :-)
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jrudisill
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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by jrudisill »

Hi Joe. This old brain has gotten very confused while trying to set the pots on the VU Buffer Board with Peak LED, used with the 34mm (small) meter. I'm using these in a single Langevin AM-16 mic pre racked with a 40 dB pad on the INPUT. The meter is bridged across the output XLR. The meter and Buffer both seem to work fine.
I'm assuming the pad should be set to zero when setting the levels for the pots, first with +4dBu OUT to set the meter; then+18 dBu OUT when setting the peak LED (I disconnect the meter while adjusting the LED pot). When I run a Shure SM58 into the input and monitor with headphones, the meter reads way too low, and of course the Peak LED is dark, no matter how hard I overdrive the mic.
I've tried several other options, including converting dBu to dBm, but nothing is making sense – a sure sign of senility onset.
Here's some background details:
PS – AMB Labratories Sigma 11 wired for +24 vdc output (https://www.amb.org/audio/sigma11/ )
+48vdc derived from PS using an XP Power DC/DC step up converter.
Level measurements made on a very old, uncalibrated HP 400E AC voltmeter with dB scales.
Input from computer sig gen since my Wavetek 180 died on me.

There was a separate phenomenon, not related to the above, but worthy of mention. I first connected the output signal from the XLR to the HP 400E meter using a cable with BNC connector (2-wire), not realizing that pins 1&3 were common in the female XLR connector. After I noticed there was no indication of power from the PS, and the power xfmr was too hot to touch, I traced the problem to this cable with pins 1&3 common if the 34 mm meter and buffer were across the output. Disconnecting the meter/buffer corrected the problem, though I can't figure why. Using a cable with pin 3 of the XLR open also resolved the problem. Any idea what could've caused this or how to prevent it from occurring again?

Your advice and help are greatly appreciated.

Jeff Rudisill
jeff@jeffrudisill.com

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Re: VU BUFFER & PEAK LED KIT BUILD THREAD

Post by Joe Malone »

The output level you need to set the meter buffer to will depend on the input level to your A/D input level which should be a pro +18dBu to +24dBu device to suit the full output of the AM16. What A/D is this going into? Using a amp and headphone with no input level reference will no help calibrate this to your audio setup.

A 40dB pad in front if a AM-16 device makes it basically a unity gain device which will be useless for a mic preamp. If you are using it to boost a mic then 40dB gain without a pad is near perfect usually to get +4dBu.

Make sure the centre taps of the input and output transformer are not connected to anything. They sound like the output has its center tap connected to 0v. And if a pin1&3 shorted cable are plugged in it will short half of the output transformer making it over heat when driven by audio. The buffer if built correctly cannot cause the overheat problem.

Otherwise as always photos of your wiring will usually help be find the problem.
Joe :-)
JLM Audio
Capturing Audio without Injury

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