| Geek |
| You'll need more gain if you use a dynamic mike. Try a 470k resistor in the pentode plate and adjust the cathode resistor accordingly for bias. |
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| SY |
| Unavailable and pretty noisy. |
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| SY |
But she's still a noisy (for this app) pentode.
Thanks for the correction- owners of stock ST70s will be delighted. |
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| fdegrove |
Hi,
..........And why use a triode/penthode where a single LOW noise penthode would do nicely already?
Isn't there a circuit using the 5879 in one of the RCA Receiving Tube Manuals designed for micpreamp use.......?
Or an EF86/6267?
At least that one's still around in abundance and likely to stay with us for another twenty years.
Plenty of low level circuits available to pick from as well.
No idea about the US but in Europe at least there are still some absolutely amazing small signal penthodes around that are hardly used by anyone anymore.
Quite a few of those could be just the ticket you need.
Cheers,;) |
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| Sir Trefor |
I can either try Geek's Idea(which I don't have a problem with), or go with fdegrove's idea, but there are no other circuits like this in my manuel.
Does anyone in this thread hav a schematic of a circuit that would work better? A single pentode?... What about phantom power?
Thanks |
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| Sir Trefor |
Cool! Phantom power :D!
-Trevor
PS- I was thinking...could I replace the one 7199 tube in my circuit with 2 tubes (a triode and a pentode) that would provide similar gain characteristics? Before I even posted this thread I had heard unfavorable comments about the 7199... |
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| Geek |
| Less tubes = less noise. A dual triode will be less noisy than a triode/pentode and a single pentode (like a 6AU6) can be less noisy than a dual triode...... |
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| Sir Trefor |
what about a 12AX7?
-Trevor
PS- Geek, is the phantom power circuit you posted supposed to be it's own console, seperate from the preamp? And I assume that I am reading the schematic from left to right---- with the input being the jack on the left side of the drawing, and the right being the output. correct? |
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| Geek |
| quote: | Originally posted by Sir Trefor
what about a 12AX7? |
12AX7 is an excellent tube. Using both sections will give you plenty of gain, but without NFB, will be noisier than a single high-gain pentode.
| quote: | | PS- Geek, is the phantom power circuit you posted supposed to be it's own console, seperate from the preamp? And I assume that I am reading the schematic from left to right---- with the input being the jack on the left side of the drawing, and the right being the output. correct? |
You can place it anywhere you want. In the preamp or mahe an inline coupler from a plumbing pipe and some connectors.
Yes, I forgot to add that in the circuit, the DC blocked side to the preamp, the mic to the other. Reverse it and your preamp frontend may go POOF! |
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| fdegrove |
Hi,
| quote: | | Using both sections will give you plenty of gain, but without NFB, will be noisier than a single high-gain pentode. |
One of the most amazing small penthodes I know of is the D3a.
Even trioded it still has a mu of 77 and a Req of 65 Ohm.
I have no idea it's ever been used in a mic pre but I'm sure it could.
Cheers,;) |
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| Geek |
Hey Frank,
Sounds like a candidate for a phantom powered, in-mic pre, eh? :) |
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| fdegrove |
Hi,
| quote: | | Sounds like a candidate for a phantom powered, in-mic pre, eh? |
Ah...Guess you had a look at the datasheet.
Among other apps, yeah...Would be great.
You can still find original Siemens NOS for very reasonable money.
Cheers,;) |
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| moamps |
| quote: | Originally posted by Geek
Phantom power injector :D
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Hi,
Standardized value for polarised resistors in balanced phantom power supply is 6k8 (not 10k). Manufacturers design mic's electronic using this value. Also, serial caps are to small; input mic preamp's impedance is few kohms. Usually, serial caps are good quality elcos 47uF/63V blocked with small (100nF) MKP.
Regards
Milan |
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| Geek |
Indeed, you are correct!
I couldn't remember the exact values when I ran that off and mine is glued shut. |
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| Sir Trefor |
I will now have to go back to the electronics store to purchase the nessasary parts; as I am already in the process of building this thing---- thanks for the warning; at least I didn't solder anything together yet!
-Trevor
PS- thanks, everyone, you've given me me great info on what tubes I should use--- but I need a circuit schematic to use the tubes in! |
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| Sir Trefor |
| quote: | Originally posted by moamps
Hi,
Standardized value for polarised resistors in balanced phantom power supply is 6k8 (not 10k). Manufacturers design mic's electronic using this value. Also, serial caps are to small; input mic preamp's impedance is few kohms. Usually, serial caps are good quality elcos 47uF/63V blocked with small (100nF) MKP.
Regards
Milan |
Do you mean 6 to 8k..., or... 6k?
which caps are you referring to? (what are the serial caps)? And, when you refer to "blocking" the serial caps with smaller caps, does that just mean to connect the 2 caps in series? |
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| electronaut |
| quote: | Originally posted by Sir Trefor
Do you mean 6 to 8k..., or... 6k? |
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. |
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| Sir Trefor |
| quote: | Originally posted by electronaut
Hope this helps
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Very much so! |
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| Sir Trefor |
| quote: | Originally posted by Geek
the 12AX7 is an excellent tube. Using both sections will give you plenty of gain, but without NFB, will be noisier than a single high-gain pentode.
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hate to sound like the rookie that I am... but what is NFB? |
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| fdegrove |
Hi,
Negative feedback.
Cheers,;) |
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| moamps |
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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| Sir Trefor |
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. |
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| electronaut |
| quote: | Originally posted by Sir Trefor
I need a 4 channel mixer with a mono output, and a 4 channel mic pre with a mono output. |
Oh man... that's terrible!!!
If I were you I'd get something like this. |
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| Sir Trefor |
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? |
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| electronaut |
| This guy has every manual known to humankind. |
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| Sir Trefor |
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 |
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| Sir Trefor |
| 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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| Sir Trefor |
Finally got my picture hosted so you guys can see it- phantom power injector .
As you can see I used no circuit board.
This is what my "desk" (or "cardtable";)) looked like after completing it-
click the link!
Like mentioned earlier, all my equipment was stolen briefly after I finished this, so I haven't tried it yet.
Oh, yeah... and I used essex magnet wire to connect everything inside. |
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