• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube mic preamp/mixer

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#21
Sir Trefor said:

Do you mean 6 to 8k..., or... 6k?
Click to expand...

In every application I have ever seen, Phantom power is supplied through precision matched 6.8K resistors. (6.81K is the value I've seen in catalogs -- the important thing is that they are matched to each other so that there is no DC imbalance between + and -)

I believe the caps moamps is referring to are the .1µ caps shown in the balanced XLR part of the schematic, which block the DC from entering the preamp. Depending on the input circuitry of your preamp, they may or may not be needed. Some input transformers can handle the DC -- as Per Lundahl pointed out to me once, "What is important is the NET voltage at the transformer primaries. With perfectly matched phantom resistors, the net voltage will be zero."

Then there's the old Telefunken trick: Use a DPDT switch for the phantom. One pole energizes the phantom resistors, while the other pole inserts a 10µ cap in between the primaries of the input transformer. (Only works if the input transformer has two sets of primaries connected in series.) When the phantom power of off, a wire shorts out the 10µ cap, taking it completely out of circuit.

If the input circuitry isn't exactly known or easily traceable, I'd say stick with the blocking caps to be safe. I do agree with moamps that they should be a higher value, though, and they should be good quality! (They're right at the beginning of the signal path!)

Hope this helps,

-E.
 
#25
In attached picture is one example of input circuitry in one PA mixer's mono module. Maybe helps too.
Electronaut, thanks for assistance.🙂

Regards,
Milan
 

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#26
sadness...

Alright, I've got my phantom power box nearly finished(I will post pictures when I am done). But, now it appears that my mic pre and mixer projects have just gotten a little more important----
Both of my Behringer mixers, along with my Sennheiser microphone, and all of my cables were stolen out of the trunk of my car. I now have nothing to record with. I need a 4 channel mixer with a mono output, and a 4 channel mic pre with a mono output.
 
#28
Looks like something I could really use... Does anyone have any detailed information on it? I googled it and came up with about 1000 web pages that only popped up because the word "Altec" was mentioned on the page once, or something--- you know how it goes.

-Trevor

:edit: I also tried searching for a manuel of it on ebay- I found none. Can I get a reprint manuel somewhere?
 
#30
Everything is fine

Ok, so I am currently bidding on the aforementioned Altec 1567 preamp/mixer.

I am also working on my phantom box, but I just realized something--- as I study the schematic I notice that the capacitors at the beginning of the voltage source(220 uF) are polarized. Why do they need to be polarized?

-Trevor
 
#31
Success! (sort of...)

I have completed my phantom power unit, but there is soemthing wrong... when I first completed it and powered it on, I had +48V total (24V+24V). When I tested it a few hours later I was measuring only +40V of power (20V+20V); what could cause this?
 
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