External Sound Card for Audio Analysis?

[Scott Richardson AA6U]

I finally decided to go with a laptop (PC) to analyze preamp and power amp performance. The only issue I am facing now is how to get a good audio signal into and out of the laptop. The laptop has only a combo headphone/mic jack which I doubt is very good in the way of sound quality and I think it only has mono input. Therefore, I believe that the best way to go is to get an external sound card that will plug into a USB port and have two-channel input and output.

Because I have not experience with sound cards, I am looking for recommendations for an external sound card that will output stereo signals from the laptop's audio analyzer software and provide stereo inputs into the laptop so I can analyze the output from stereo preamps and power amps using a dummy load. That means that the sound card input will need to be able to accept 600 ohms from a preamp output and 8 ohms from a power amp output.

I don't have tons of money, but I want something that will give me reasonably accurate results for a reasonable price. I work on audio equipment as a hobby, so I don't want to invest a lot of money into this.

Suggestions?

Thanks,

 

[Doodski]

Are you sure you don't want to get a SDS1202X-E200 MHz Siglent O-scope for ~$350.00 It has advantages and doesn't require fudging anything.

 

[Scott Richardson AA6U]

That is probably overkill for how much I'll use it. I have a decent oscilloscope now. None of the measurements I'll be doing needs laboratory accuracy. I'm repairing/refurbishing older tube equipment for a hobby. 1960s McIntosh and Marantz tube units are probably the highest end stuff I do.

 

[DVDdoug]

What you want is an audio interface. Interfaces with switchable mic/line inputs are common.

That means that the sound card input will need to be able to accept 600 ohms from a preamp output and 8 ohms from a power amp output.

Impedance isn't a problem. Line inputs are typically 10K - 100K and it's normal to run a low-impedance output into a high impedance output. (We don't do "impedance matching".)

However, you probably do want a dummy load on your power amp.

And the voltage will be an issue with anything other than a a few Watts so you might need to make one or more voltage dividers.

 

[Doodski]

I'm not familiar with working on audio tube amps. What kind of voltage and current are typically involved? Is it lethal zone?

 

[gene_stl]

There are two solutions I know of for the voltage problem.
One is the Linear Audio gadget:
https://linearaudio.nl/la-autoranger

The other is the millett kit:
http://pmillett.com/ATEST.htm

because of my age building either of these things is not terribly appealing but the quantasylum distortion analyzer is but a bit more spensive:
https://quantasylum.com/collections/frontpage/products/qa402-audio-analyzer

"There may be others, but they haven't been discovered"

@Doodski you COULD get yourself kilt working on tube audio gear but I don't think it has happened very often, in the absence of a heart condition.
You DEFINITELY will get some memorable shocks though, when you don't allow enough time for power supply caps to discharge. The lethal zone is more common for ham radio transmitters and linear amplifiers which can get into the kv ranges. There are a few audio nuts that do this notably the you tuber el Paso tube amps but he is an engineer from the tube days and does know what he is doing. But I myself used to draw the line at about a 75 watt tube power supply.

THIS is a tube amp:

When I was young and skinny a friend of mine(also an older engineer who had built over powered ham transmitters) wanted to give me this monster power supply he built to drive two Dyna Mk III chasis and a Lafayette dual triode tube preamp. I had it for a while but gave it back because I was scared of it and was never going to work on it. It made scary noises when booting up. It WAS a thing of beauty.

My possibly poor recollection is that KT88 output amps have b+ supplies in the 700 volt range. I think that is the worst bite I ever got back in the seventies when there were more tube amps floating around.

 

[Scott Richardson AA6U]

Yes, it looks like what I need is an audio interface. Note: I have been working on tube amps and preamps for years, so I'm very familiar with working on them (yes, some do have lethal voltages). However, I thought I'd advance into the modern age in analyzing performance by using a computer. I have been using analog analysis equipment from Potomac Instruments, Leader and Tektronics.

 

[MakeMineVinyl]

I'm not familiar with working on audio tube amps. What kind of voltage and current are typically involved? Is it lethal zone?

Power supply voltages can massively hurt if contacted. Don't ask me how I know.

 

[Doodski]

So a person operating test gear really needs to be setup properly for tube work. High voltage probes and leads etc.

 

[Scott Richardson AA6U]

Regarding the voltage issue, I am running each channel of the power amps into a non-inductive 8 ohm dummy load for testing. Could I use an L-pad connected to the output of the dummy load to control the voltage into the audio interface unit?

The voltage control devices mentioned in a previous post are rather pricey or look like a lot of work to build. I'd like something economical and ready to use if possible.

 

[gene_stl]

The trouble with L pads is that you will make a misteak and blow up the sound card and possibly the computer it is connected to. That is why those automated attenuator things were invented. However L pads will work until the mistake happens.

Most mutimeter test leads have sufficient insulation that you don't need a high voltage probe. Whatever shocks I got were not related to test lead insulation , but rather to not being careful enough. It makes you much more careful. Another thing to like about solid state amps.

 

[LTig]

Power supply voltages can massively hurt if contacted. Don't ask me how I know.

A colleague of mine once touched the supply line of a 1 KW RF tube power amp (2.4 kV DC with 1 A current delivery). She was found in the other end of the room - luckily alive.

 

[LTig]

I'm not familiar with working on audio tube amps. What kind of voltage and current are typically involved? Is it lethal zone?

YES. Always keep one hand in your pockets so chances are lower that current runs through your heart, thereby stopping it.

 

[gene_stl]

I had forgotten that one. Good advice.

Tube supplies can bite hard even when switched off. For RF Transmitter usually in the door of the relay rack enclosure there would be a grounding lead and long wood handled stick to make sure everything was completely discharged. I never worked on that stuff but I did work on some electron microscopes which have some fairly scary power supplies too. Oil filled tanks. Sometimes they arc anyway and even blow up among other interesting things they do.

 

[Scott Richardson AA6U]

Here is a manually-switchable voltage limiter that is the type of project I would have time to build:

https://www.akitika.com/documents/BuildingTheAttenuatorRev4.pdf

 

[MakeMineVinyl]

YES. Always keep one hand in your pockets so chances are lower that current runs through your heart, thereby stopping it.

Did that. still hurts like hell.

 

[Doodski]

Did that. still hurts like hell.

A electrocution to specific muscle zones can cause rapid deflection; enough to break stuff or even eyewear in one case. After the first really good electrocution deflection I've learned to use the proper test leads and clips if possible. Probes are OK but clips get a better measurement anyway. That's linear DC supply and amp what about tube voltages. Do tube repair peeps even meter the unit while it is powered ON? Are there calibrations requiring power ON? How often are these higher voltages being serviced?

 

[Scott Richardson AA6U]

Interesting how this thread has morphed into a different subject entirely...

 

[MakeMineVinyl]

A electrocution to specific muscle zones can cause rapid deflection; enough to break stuff or even eyewear in one case. After the first really good electrocution deflection I've learned to use the proper test leads and clips if possible. Probes are OK but clips get a better measurement anyway. That's linear DC supply and amp what about tube voltages. Do tube repair peeps even meter the unit while it is powered ON? Are there calibrations requiring power ON? How often are these higher voltages being serviced?

Yes, some voltages have to be measured while the unit is powered up. Luckily, tube gear is pretty reliable so venturing into them is not an everyday thing, at least for me. One thing to remember to do is to discharge all capacitors which potentially have a high voltage charge, as they can take a long time to self-discharge. Because of the high voltages, there is a lot of energy stored up in those caps, so discharging them with a screwdriver is not recommended.

 

[gino1961]

Hi very interesting thread I guess that a good start could be a usb sound card with a very low noise ?
if so the problem is to select one
For SW i like Arta The demo version is free