Tube Tech MP1A: Phantom power crapping out (solved)

I'm trying to fix a Tube Tech MP1A preamp for a friend of mine. The problem is that the voltage drop for condenser mics using 48V phantom power is way too much. The most extreme case is with an AKG C414, which drops down to about 2V when phantom is engaged. With no mic in either input, the voltage is at 49V. In fact, if I engage phantom on both channels, but only plug a mic into one channel, the unused channel still shows 49V. The voltage for the C414 drops down to about 35V with every other preamp I've tried, so I know it's not the mic.

I got the schematics from Tube Tech, and here's the power supply:

You can see the phantom section there in the middle.

However, what I've actually got on the PCB is this:


As you can see, the path is different, as well as the value of C26.

So, would this discrepancy cause the issue that I'm having? Or should I be looking elsewhere?

Did u ask tube Tech?

Try making it like the original schem. Seems they f----d up good.

I've got an email in to their US rep and I'm waiting on a reply. I'm guessing that this particular power supply board is between revisions. I've tried about 10 different condenser mics and most of them have worked fine. The only ones I've had a problem with are the C414 and an Audio Technica 4033a. The 414 is unusable, as there is very little output and it's extremely distorted. The 4033a distorts at high levels.

I can't think of what that bottom one would do, but if you look at any standard reg setups it is very wrong by comparison.

What I think is going on with the LM78L12 is that there's that 36V Zener diode at the ground leg, which is setting a virtual ground reference at +36V. Combine that with the regulated 12V from the LM78L12, and you've got +48V.

The LM7815 below it in the schematic has the same layout for it's first cap as my corrected drawing. 15V above the +18V virtual ground yields +33V. So, which is right?

Go back to basics first. Ohms law current = voltage divided by resistance

Make a number of different resistor loads to test at different currents. This will help find the spot the place the phantom goes to current limiting

Remember to take into account the phantom resistors

Take measurements at the input output and ground(zener) pins of the 78L12 and the emitter and base of Q4 this will give a good idea of what is wrong

I would suspect Q4 and R50 bias do not supply enough current at higher ma draw.

Here's what Tube Tech just sent me:

The +48V phantom power is fed to the XLR socket via two 6K81 resistors and
for the voltage to drop to 2V, it means that there is a current draw at
about 13mA which is very high for a microphone. To me it sounds more like
there is a short to ground in the circuit from the XLR to the mic. or that
these resistors are much higher than 6K81 or maybe a bad contact in the
switch (then it really has to be bad).
I take for granted that the +48V, when measured on the PSU-PCB, doesn't
change.

In MP 1A serial no. 5939 and later (PCB 870922-5 and later) the phantom
power is like your drawing, but it doesn't matter for the function of the
circuit. The cap. is only to avoid any oscillation. If its connected to
ground or to the ground reference of the IC has the same function.

I have checked the current consumption of the C414 and its approx. 4,5mA, which will result in a voltage drop of 15V.

Click to expand...

The one thing I haven't checked is the 6.81k resistors at the inputs. If they were mistakenly stuffed with a higher value, that would explain what I'm seeing here, especially since the voltage at the output of the power supply PCB is a constant 49V, regardless of load. I'm going to check those out this evening.
I'll also check out what you suggested, Gus. I've already checked R50, and the value is fine. I've got a replacement Q4 already installed, so maybe that will solve the issue when I fire it up.

Thanks for the suggestions.

PROBLEM SOLVED:

All four of the phantom resistors measured 68.1k, instead of 6.81k. The multiplier stripe on the resistor is definitely not orange, but it's also not as red as the other red stripe on the resistor. I could easily see how somebody would make the mistake when they looked at the color code.

wmtunate,
Remember you need 0,1 % tolerance resistors at this place !
Guy

[quote author="guy_4"]wmtunate,
Remember you need 0,1 % tolerance resistors at this place !
Guy[/quote]

Got 'em. I had some from an API 312 build that I haven't finished yet.

[quote author="wmtunate"]PROBLEM SOLVED:[/quote]

Please let Tubetec know this problem - with your unit's serial number - I'm sure they would like to know this potential problem for future troubleshooting..

Jakob E.

I'm sure that the Company would want to know about this problem, it doesn't seem from the reply you had from them that they were aware of this before.

It is hard to think that you have the only one with the incorrect value resistors fitted (Tube Tech will be quite worried now) - but they will have to change their final test procedure to ensure that Phantom voltage is measured under load.

Well done!

I sent them a message last night right after I found the problem. They've been more than helpful throughout our troubleshooting and they sent me a response this morning:

Doesn't look good.
When looking at the stock status of 1999, it is likely that a batch of these
MP 1A was manufactured. There was too many 6K81 and too few of 68K1.
I'm surprised that no one has complained of this before.
I will see if I can find them, might be difficult after almost nine years.

The MP 1A in question is probably: 07251-07275

By the way, the multiplier on the resistor (1% tol.) shall be brown for the
6K81 (blue, grey, brown, brown) and red for the 68K1 (blue, grey, brown,
red)

Click to expand...

This MP1A is serial no 07263, so it's right in the middle of this batch. According to Tube Tech, it was manufactured and tested on July 30, 1999.

Hi,
i'm trying to understand this psu and have some questions;
1. What is 33V after R49 doing? Is it connected to anything?
2. Why heater supply have 33V? Also, voltage that enters to LM7815 seem to be very high, wouldn't regulator get hot this way?
3. Is it possible to make B+ and heater supplys variable somehow?

Miha

> an AKG C414, which drops down to about 2V when phantom is engaged. ... The voltage for the C414 drops down to about 35V with every other preamp

> phantom resistors measured 68.1k, instead of 6.81k

Right. AKG 414 at idle draws a constant current. I forget 1.4mA or 2mA; assume 2mA, 1mA per leg. Then in 68K it wants to drop 68V. Since it started from 49V, it can't; anyway the 2mA current limiter needs ~~2V keep-alive, so it settles to 2V and not working right. With 6.8K it drops 6.8V, or from 48V open open to ~~41V. (Or 24V open to 17V loaded; it is specced to run below 12V.)

That's for my circa 1991 AKG C 414 B-ULS. Your 35V on happy phantom suggests your model is pulling 2mA per leg, 4mA total. What-ever.

It is mildly astonishing that TT didn't have a flood of complaints; most all phantom mikes will starve on 68K.

> Remember you need 0,1 % tolerance resistors at this place !

AKG requires 0.5%. I know it works fine with clean 24V and unmatched 4.7K 5%. A "need" for 0.1% implies a pretty sloppy phantom. Yup, there's some sloppy stuff on the market.

And not "tolerance", matching. 6K or 8K will work, as long as both in each pair are similar enough that common-mode phantom crap balances out, and DC unbalance does not soak the transformer inside the mike (my 414 tranny is near as big as a tweed Champ OT, so probably can stand a bit of DC unbalance).

> What is 33V after R49 doing? Is it connected to anything?

R49 leaks current from the +33V at U2 into D2. Without it, the current in D2 is just 30 microamps, and most Zeners are not designed to run that cold. We could reduce R46 to flow more down to Q3 and D2, but that would cause Q3 base to load R47 R48 more. We could leak current from +270V, but it is wasteful to drop 270V to 28V. A 28V Zener would like several mA, and that could be about 1 Watt of waste. Instead R49 leaks 4mA down from +33V, 20mW heat, and a more stable source too.

> Why heater supply have 33V?

They probably run 6V and 12V heaters in series at 0.150 Amps.

> voltage that enters to LM7815 seem to be very high, wouldn't regulator get hot this way?

It isn't the total voltage, but the voltage actually "felt" by the device, AND the current.

The 35VAC is probably 48V DC. D9 takes 18V of this, the chip sees 40V. (Which is indeed right AT the rating.) The output is 33V. The chip drops 48-33= 15V, at 0.150 Amps, 2.25 Watts. Assuming it has some hunk of metal bolted on, it is fine. (And if not, it will shut-down.)

> Is it possible to make B+ and heater supplys variable somehow?

Yes. This is the magic of Electronics. EVERYTHING can be varied.