Input Impedance of Soundcard

If I've asked this B4, forgive my alzheimers.... couldn't find it thru search.

I think New York Dave had a simple pre design where he mentioned that if you're going into "modern equipment" you can skip the OT. Slowblow avoids the OT like the plague. The basic Jensen circuits skip the OT. Then I see the other designs, where the OT appears critical.

In terms of my soundcard, if I go Mic/Valve Pre/Card, what is the likely impedance the soundcard is looking for ......... big mismatch or OK?

Sound card inputs are usually about 10K. For practical reasons you should drive them with less than 10K. That means most transistor preamps can drive directly, also most transformer-coupled tube amps, but not tube amps without output transformers.

There are exceptions, though: tube preamps with a cathode follower output stage, or common-cathode outputs run at a high-ish plate current with negative feedback, may be able to drive 10K loads with no problem. It really depends on the circuit, but it's true that tube circuits are intrinsically higher-impedance and you're more likely, in the general sense, to have trouble driving low- and medium-Z loads without a stepdown transformer.

I'm presently building a version of my simple tube preamp design with no output transformer. I intend to use it with my multitrack, which has an unbalanced >10K input. The predicted performance into loads of 10K and greater is good, but I'll let you know if the real-word measurements agree.

If I were building this for someone else, I would probably include an output transformer and accept the 12 to 14dB loss in voltage gain, just because I could never be sure that somebody wouldn't try to run it into a load below 10K. But in my case, I need every dB of gain (I wanna use it with a ribbon mic) and I'm going to be the only person using it, so I don't really need the OT.

Thanks heaps and heaps...........................so with something like the Ampex 351 circuit, what Z/ratio should I aim for? Can it afford such a loss of gain from adding a X, or am I likely to need more stages.

> something like the Ampex 351 circuit

Is that the real question? Or just a random thought?

The 351's output IS transformer, low-Z, and no easy way to bypass the transformer. And no good reason to do so. Therefore you just adapt its XLR output to your sound card input, done.

If you have some hot-rodded 351, or something "like" a 351 only different, or something else altogether, you better say what you really have or you will get irrelevant advice.

Woops...thanks for pulling me back. I need to be clear.... I don't have a modded 351.

I'm considering scratch building something like the 351 recording circuit, ie input tranny, 2 triode stages, then OT if needed...... to unbalanced out. Looking at the schematic I have for the 351, the output to the record board doesn't show a transformer, so I assumed it didn't use/need one for tape..................but maybe needed one for soundcard input..... Have I got my wires crossed?

If I use an OT, I'm just wondering about impedance ratio ie turns ratio.

from NYD:

But in my case, I need every dB of gain (I wanna use it with a ribbon mic) and I'm going to be the only person using it, so I don't really need the OT.

Click to expand...

Sounds like we have an quite alike setup: the budget ribbon-mic (you from Fum, I from Thomann) and both the VSR-880. I might be interested in the thing in between as well ! :wink: :thumb: Interested to hear about your experiences when there's more news.

BTW/FIW, about those VSR-880-inputs, I did some measuring and found they're some +11.4dBu max (at the default ATT-setting of 0 dB; you know it can do +/- 12dB). IIRIC the input-Z is 16k.

Thanks / regards,

Peter

OK .... I see from my book that the 12AX7 has a plate resistance of 60k-80k approx depending on voltage. My soundcard has an input Z of 10k. Low current though, the 12AX7 is running .5 to 1.2 ma........... but still...it has a plate dissipation of about 1watt. That would seem a decent pushhhhhhhhhhhhhhhhhhhhhhh ................................................... ....................................................... ..................................................... look forward to hearing more when you're ready. Thanks for your response so far!

> the 12AX7 ...has a plate dissipation of about 1watt. That would seem a decent pushhhhh.....

It takes a heck of a lot of voltage to force 1 Watt through a 12AX7. Mostly they run about 0.060 Watts, like this one.

> ie input tranny, 2 triode stages

In the Ampex 351:

input iron: 23dB, 1:14

1st tube: 21.5dB, 1:12

2nd tube: 22dB, 1:12 into 250K
estimated -6dB into 10K load

total gain before rec/play switch: 66.5dB, 1:2,000 into 250K
estimated total gain before rec/play 38.5dB into 10K load

line output stage: 25.5dB, 1:19

total available gain: 92dB, 1:40,000

So there is a lot of voltage gain after two triode stages, but only for high (250K) load. 38dB into 10K is not a lot of gain.

Also the 12AX7 is running only about 0.5mA. If you have inputs like Peter's, they need 0.25 to 0.5mA peak (3VRMS in 10K-16K). You can get almost 0.5mA peaks from a 0.5mA amplifier, but it will be grossly distorted.

At a minimum, I think you want a cathode follower to present >250K to the 2nd stage, and flowing 2mA or more to give solid drive to a 10K load. With 12AX7, that probably means 200 Volts across the tube. You might be looking at 12AU7 or other fatter tube. Overall gain is 66dB.

You will always (in audio) get more power gain with plate-loading. The existing 12AX7 is unsuitable: you can't wind a good transformer for a 60K source. And if you could wind 250K:10K, that is a 5:1 or 14dB loss of voltage gain, just 52dB overall gain. What you do is follow the 2nd stage with a low/medium-Mu tube like 12AU7. Even then, the plate resistance (about 6K) is high for a transformer. For good triode loading we want over 10K primary. The sound card may have radio supression caps that expect less than 1K source. 10K:1K is 3:1 voltage. The 12AU7 will give nearly full gain, say 18. The tranny gives 3:1. Overall gain is 1:6 or 15dB. Input impedance is high, so with the first two stages we get about 66+15= 81dB gain. A naked 12AU7 with transformer working at line level is not dead-clean like the 12AX7 working at lower levels: that's why Ampex went to a 2-stage push-pull line amp, so they could get some feedback. (And so they could drive true 600Ω loads.)

There is more gain in the 351 electronics. The first two stages are heavily socked with local feedback. Ampex was not building a Fender. You could throw some caps in there and get another 10dB-12dB gain. But now it isn't an Ampex 351. Do you want cheap or do you want good? Tube amps always want one more stage than you budgeted for. (No matter how big your budget started.... look at the McIntoshes and the Audio Research amps.)

Wow, that's a very comprehensive response. Thank you.... I've learned some good things.

It makes sense to me... I didn't realise the input impedance of the tape player was that high, relative to the soundcard. I can see there is some serious negative feedback through degeneration at the cathodes of the 12AX7. These could be bypassed by capacitors, which would push the gain up a lot. Changing the cathode resistors would push up the current. But I'm getting into the design field, and I clearly don't know enough.

There are some great projects already sorted or being sorted.....might be smarter if I look at those.

Thanks for the info and patience

If we're talking about a 10k input impedance, unbalanced, -10dBV nominal sensitivity on the sound card, you can usually drive that quite happily with a voltage amplifier tube direct-coupled to a cathode follower. A 6SL7 and a 12AU7 make a fine combination even without feedback, or you can enclose the pair in a feedback loop as though it was an inverting opamp (remember to take the feedback from the right side of the output coupling cap!). The only hassle is that, with or without feedback, the circuit does invert polarity. You may be able to make that right somplace else in the setup.

Peace,
Paul

See the "One-Bottle Preamp" thread over on the Lab...

The last thing in my signal chain before the soundcard is a pultec eqp-1a - should I terminate the eq with a 600 ohm resistor?

What a ghost town this place is. Bump.

Really nice, btw:

http://www.ghosttowngallery.com/htme/cimarron.htm

The short answer is that if it has a XX:600 output transformer, then yes, the output should be terminated with 600 ohms if you want the flattest response from the transformer. But try it with and without--you may actually prefer the sound from the "incorrect" way.

You can use a 600 ohm resistor and a switch like the Loading switch in the Great River preamps. This way you can have both types of sound and you can choose what you like for a specific situation.

chrissugar

I terminated the eq and I notice that things sound a little tighter, possibly due to a loss of some presence in the highs.

Not sure I like it.

Anyway, it appears that it's not necessarily bad practice to go without termination in this case.

Thanks.

Hi I use two Gina card (Echo Audio). I suppose it has (like similar card) 10K input.
In my case it is unbalanced input. What to do with 600ohm out equipment. Just to terminated with 620ohm XLR pin2 and pin3 or anything else? Balanced to unbalanced connection I made wiring pin 3 to pin1.
I think about Neve, Api, Langevin, Pultec, LA etc.
Thanks
Duka

[quote author="dukasound"]Hi I use two Gina card (Echo Audio). I suppose it has (like similar card) 10K input.
In my case it is unbalanced input. What to do with 600ohm out equipment. Just to terminated with 620ohm XLR pin2 and pin3 or anything else? Balanced to unbalanced connection I made wiring pin 3 to pin1.
I think about Neve, Api, Langevin, Pultec, LA etc.[/quote]

If the input's 10k and you want to be really picky, terminate the previous stage with a 634 ohm resistor rather than 620; ideal is 638 to create a total 600 ohm load when paralleled with 10k.

Peace,
Paul

[quote author="pstamler"][quote author="dukasound"]Hi I use two Gina card (Echo Audio). I suppose it has (like similar card) 10K input.
In my case it is unbalanced input. What to do with 600ohm out equipment. Just to terminated with 620ohm XLR pin2 and pin3 or anything else? Balanced to unbalanced connection I made wiring pin 3 to pin1.
I think about Neve, Api, Langevin, Pultec, LA etc.[/quote]

If the input's 10k and you want to be really picky, terminate the previous stage with a 634 ohm resistor rather than 620; ideal is 638 to create a total 600 ohm load when paralleled with 10k.

Peace,
Paul[/quote]
Thanks Paul.
What with my dbx 286 preamp who have 200ohm balanced output. Does it mean that we must to terminate onlz 600ohm output.
Best
Duka