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Question

Input impedance mismatch of preamp?

by Mikey_210 / June 4, 2013 10:54 AM PDT

Audio-Technica AT120E phono cartridge into a PS Audio IV preamp... I'm getting really poor gain and flat, muffled sound. A/B testing, a Carver preamp works fine. (That unit has its own issues and I'd like to obsolete it.)

I got the PS Audio unit used, so I'm guessing there's an impedance mismatch at work here. Does anyone know how to troubleshoot this, or a resource for guessing how to tweak the PS Audio IV preamp?

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All Answers

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Answer
Phono cartridge outputs
by mjd420nova / June 4, 2013 11:23 AM PDT

Phono cartridges usually have an impedance of around 100K ohms, where an auxillary or line input would be around 10K to 20K ohms. Preamps specifically made for phono use will provide the best match to the input and thus much more content (voltage level).

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Phono preamp:
by Mikey_210 / June 4, 2013 12:17 PM PDT

Thanks, Mr. nova...

The PS Audio IV preamp was built in the 80s, so it most definitely knows phono. It's feeding a boat-anchor amplifier and Acoustat 2+2, eight-foot electrostatic loudspeakers. When I hit the right impedance match, I'll know it. (So will my neighbors.) Rock and roll.

I'm going to pop the cover and have a look...

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No, that's not how it is.
by Oldartq / June 4, 2013 1:58 PM PDT
In reply to: Phono preamp:

Of course you can said it's impedance mismatch but in truth..if this is a magnetic cartridge, then you need that extra stage of preamp, it's as simple as that. In the old day all amplifier have that. But now I see lots of record player have the preamp built-in. instead.

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Preamp:
by Mikey_210 / June 7, 2013 11:01 AM PDT

Mr. Oldartq— The PS Audio unit has a very nice preamp in it, which is why I'm flummoxed by the current issue. It was built in the 80s. I'm wondering if the previous owner had some different type of phono cartridge than the AT120E I'm running, and modified the unit or something...?

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They do have standalone preamp in the market place,
by Oldartq / June 7, 2013 5:59 PM PDT
In reply to: Preamp:

but they are not cheap though. They know it take a special breed to listen to records nowaday...and they know they are willing to pay big bucks. I see audiophile equipment goes in the thousands. Yeah..just to listen to records.

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Phono preamp:
by Mikey_210 / June 8, 2013 10:58 AM PDT

Mr. Oldartq— Indeed. I have two preamps, but I want to use the PS Audio unit if I can figure out what's goofy. Since the Carver preamp works fine, it must be the PS Audio unit, so I guess I'll have to pop the cover and have a look around inside.

This is, in fact, part of a project to digitize some rarities from vinyl to AIFF. I've been looking at ADCs (analog-to-digital converters) but I am tempted to just use the one built into my 24" iMac.

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Sound cards
by mjd420nova / June 8, 2013 11:22 AM PDT
In reply to: Phono preamp:

Creative makes some powerful but inexpensive sound cards that come with a studio mixer and effects tools. Many offer the choice to record analog inputs and convert to digital, 16 bit or 24 bit with selective bandwidths and sample rates. Some cards do offer USB support and that might be a better option.

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ADCs:
by Mikey_210 / June 8, 2013 12:06 PM PDT
In reply to: Sound cards

Excellent point, Mr. nova.

A sound card that only runs AIFF (16 bits, 44.2/stereo) might be a good investment.

I'd have to look at the software options, though. I used to run a product called CD Sound Doctor II (who was their marketing genius, anyway?) and successfully AIFFed about a thousand (!) rare tracks from vinyl. The project was time-consuming, but the results were great. That software product is no longer supported, and I gave away the boat-anchor Power Mac that ran it.

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Many options
by mjd420nova / June 10, 2013 3:27 AM PDT
In reply to: ADCs:

If you could measure the impedance of the input/output of the PS unit, it would be possible to construct a matching pad. A mismatch can have some strange effects, reducing some harmonics and even creating others not in the media. Transposing is time consuming but you get to listen to all that incredible analog content. Then do an experiment, after transposing a selection, play back both at the same time and listen to the difference.

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Here's something interesting,
by Oldartq / June 8, 2013 6:33 PM PDT
In reply to: Phono preamp:

did you know PS Audio's first product (in 1974) was preamp? What model PS Audio do you have? Does it have a "phono" input?

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PS Audio IV:
by Mikey_210 / June 9, 2013 2:00 AM PDT

My unit is a PS Audio IV preamp. Its power supply comes in a separate enclosure.

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So it sound like you have...
by Oldartq / June 9, 2013 2:29 AM PDT
In reply to: PS Audio IV:

a power supply problem, or preamp problem, or do you still have the manual? Some one mention about a switch on the bottom of the unit which has something to do about moving coil. Give that a flip and see what happen. Anyway...good luck.

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Meltdown?
by Mikey_210 / June 11, 2013 9:26 AM PDT

Son of a gun, Mr. Oldartq is correct— there's a switch on the bottom of the preamp. I flipped the switch and all I got was a nasty transient, and apparently the left channel of my power amplifier fried.

I don't know what the heck is going on here, but I'm currently running monaural, and not all that happy about it. I need to take a look at this power amp now before I can go back and address the phono preamp issue.

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OUCH
by mjd420nova / June 11, 2013 1:17 PM PDT
In reply to: Meltdown?

I heard that. That hurts. Transients can really mess things up quick. A good look at the amps input circuit might reveal the input scorched. Good luck. Next time, try all the swiching with everything turned off.

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Hopefully it's just a balance control issue.
by Oldartq / June 11, 2013 4:28 PM PDT
In reply to: Meltdown?

Does it have one? I have a Kenwood that has one of those thing. If it has one of those then you got to make sure it's not set off balance too much.

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Answer
The plane has crashed into the mountain:
by Mikey_210 / June 12, 2013 2:20 PM PDT

You fellows have been extremely patient and helpful. Thank you.

Tonight I have determined that my Adcom GFA-555II power amplifier has indeed failed, apparently as a result of a transient that came through the PS Audio phono preamp unit. The amp powers up with its "thermal protection" LED lit and the left channel is dark. (Trust me, I've pumped some current through this amp for years and have never gotten it warm enough to thermal out, so something has gone "fizzle" in there.) I will attempt to evaluate and repair— I like this amp.

As for the AT120E phono cartridge, I have yet to figure out if it's a moving coil or a moving magnet, though I found a comment on an audio forum that said it was a M-M. I had poor results with both switch positions on the PS Audio unit (before I blew the amp) and a careful inspection of the preamp's internals shows no modifications or burn marks. I suspect it was just a really old, dirty switch that somehow produced a nasty transient. Damn.

So I resurrected the old Carver Receiver and am using its power amp section to drive the big Acoustat 2+2s. (AIFF > iMac > ARCAM rDAC > PS Audio IV > Carver.) I think it sounds thin compared to the Adcom unit, but at least I have tunes.

The irony here is that the Carver's phono preamp works great— it always has. I still don't know what's the deal with the PS Audio unit's phono preamp, but I might pay someone to evaluate it, as long as the poor Adcom power amp needs a service call anyway. The PS Audio unit should sound good if it's working correctly.

So there's my sad tale. I'm really not as hapless as I sound— I carried a tool bag as a field service engineer for many years, back in the day when FSEs dragged oscilloscopes through airports and would perform component-level repaitrs on proprietary scientific instrumentation. Wish me luck, and PLAY IT LOUD!

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Loved the travel
by mjd420nova / June 12, 2013 10:44 PM PDT

I know where you're coming from. I spent my early years in the field, way in the field. Eleven western states. Luggage?? How about two heavy duty aluminum cases with a real time infra-red scanner and photo equipment. A pyramid type box to transport LN2, just a gallon. Each case was 70 lbs. The LN2 was in a vented, non pressurized approved container, mounted in a tip proof, custom case. Tariffs allowed me to carry ten gallons but 4 liters would last me a week in the field. Often I had to fly in as close as I could rent a car, get the LN2 locally and drive to whatever site. Carrying a tool kit was part of the field unit for the instrument. I had to repalce an old AR Research tuner amp when it just died from old age. Final stage has suffered multiple final transistor failure and incessant fuse popping. Replaced it with an old but venerable Pioneer surround amp (analog). It has super sound but no digital inputs. It was shipped to the garage when a Yamaha RX-V373 became the starting entry into digital sound and media. It was the only way to unite the HT system and get rid of annoying lip sync failures and lack of surround as the analog output from the source (TV, DVD/CD, cable) was not passing along the decoding from HD content. The old amp had no HDMI connection.

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Update:
by Mikey_210 / June 27, 2013 4:09 AM PDT
In reply to: Loved the travel

I hear ya, Mr. Nova... my first field territory was 13 western states and western Canada. At one point I covered a territory called "ROW," which stood for "rest of the world." That was too much for me and I quit after half a year. I have lots and lots of crazy stories about trying to get expensive gear through airports for demos in the field.

Anyway, I got my power amp repaired. Both left-channel fuses to the DC rail were blown and one of the fuse holders had melted and required replacement.

I think I'll just stick to using the old Carver phono preamp and not try the PS Audio unit again...

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