Twin output 'hifi' preamp

Subject: Twin output 'hifi' preamp
From: Richard Bridge
Date: 04 Oct 10 - 03:45 PM

Where can I get a twin output buffer amp? Input preferably switchable 100mv/line level, twin outputs both switchable 100mv/line level (or +4Db)?

I want to take the preamp output of my classic Pioneer SA1000 (old but good) "hifi" amp and use one output of the preamp to feed back to the output stages of the Pioneer, and the other to go to my drawing-room powered mixing desk for when I want it REALLY loud. The Pioneer is nominally 90 + 90 RMS into 4 ohms (but my real hifi speakers rather less) but my mixing desk will run up to about 300 + 300 and the speakers safe well beyond that.

Subject: RE: Twin output 'hifi' preamp
From: Bernard
Date: 04 Oct 10 - 04:35 PM

Probably cheaper to use a small mixer - Soundcraft Folio Notepad if you can find one, as the line level inputs have switchable RIAA equalisation. The main outputs are balanced, too.

Subject: RE: Twin output 'hifi' preamp
From: Bernard
Date: 04 Oct 10 - 04:41 PM

There's a few on eBay - nice and cheap. Alternatively, the PowerPad has a pair of built-in 30w/4 ohm amps.

Subject: RE: Twin output 'hifi' preamp
From: GUEST,me
Date: 04 Oct 10 - 04:45 PM

Maplin

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 04 Oct 10 - 05:09 PM

Eh? Does the Folio have twin power amp outs - or a record out that I can run back into the Pioneer?

Subject: RE: Twin output 'hifi' preamp
From: Bernard
Date: 04 Oct 10 - 05:17 PM

Main out (bal), Monitor out (unbal) and headphone... there's also FX send/return, and 4 mic inpts with phantom... very versatile.

We used to use one at the Woodford Air Show on top of the Control Tower, because it resisted the Radar very well. One year it rained rather hard, and the poor little Folio made some strange noises and appeared to die.

I opened it up, and about a pint of water gushed out... so, clutching at straws, I put it on the dashboard of the van and started the engine. Half an hour later it was dry... and it worked perfectly!!

Sadly, we don't have that one any more, as someone nicked it.

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 04 Oct 10 - 05:25 PM

I suppose I could use the monitor out to return (unbal) to the Pioneer, and send the main out to the Yamaha.

That reminds me I think I have some DJ mixers lying around, but I don't think any have a bal send.

It's all in the reframing of the need, isn't it?

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 05 Oct 10 - 03:51 AM

The DJ mixers won't manage a balanced send. But that led me to the answer - if I can find room in the hifi cabinet for it - my larger DI box - use two channels, phono to jack from pre-out on the Pioneer into where the guitar would plug in, jack to phono from the "through" on the DI box back to the main in on the Pioneer, and long mic leads round the room to the mixer amp. Voila! Must try it soon.

Subject: RE: Twin output 'hifi' preamp
From: treewind
Date: 05 Oct 10 - 04:09 AM

Do you actually need a buffer amp?
It's quite possible the Pioneer preamp will drive both the Pioneer power amp and the cable to the mixer input. As for gain, surely the mixer has enough of that?
All you need to do is make up a "Y" cable with three pairs of connectors. Maybe a little plastic box and a tag strip from Maplins will make a reasonably tidy job of tying the three pairs of cables together. Or use a 2-into-one adaptor

At home I have a stereo amp with "tape rec" outlet which feeds cables all over the house, to 3 different destinations (one of which is a mixer, the other two are consumer stereo amps in different rooms) and it all works fine. Some of it even goes through CAT5 wiring...

Or maybe I've misunderstood the problem.

Anahata

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 05 Oct 10 - 05:06 AM

I suspect that the "pre out" of the Pioneer will not be pokey enough for a line in on the mixer, but OTT for a mic in, and that I will get hum from the long lead if unbalanced.

I already have all the "rec out" sockets in the Pioneer in use.

It's worth a try maybe.

Subject: RE: Twin output 'hifi' preamp
From: pdq
Date: 05 Oct 10 - 10:52 AM

It might help to have the schematic to the Pioneer SA1000...

                                                                            http://www.freeservicemanuals.net/file.asp?id=1388329

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 05 Oct 10 - 11:19 AM

Is that download safe? Its safety certificate does not match.

Subject: RE: Twin output 'hifi' preamp
From: pdq
Date: 05 Oct 10 - 12:39 PM

It's an old trick to use the tape monitor (out, then back in) to accomodate a graphic equalizer. Whatever is attached to the tape monitor can have more than one output.

I read about balanced line output in home stereo once in a while, but it is really not necessary. Balanced line was used in commercial installations such as theaters where the line level source had to drive amplifiers that were located hunderds of feet away, resulting in hum and loss of high frequency.

Subject: RE: Twin output 'hifi' preamp
From: treewind
Date: 05 Oct 10 - 01:49 PM

"I suspect that the "pre out" of the Pioneer will not be pokey enough for a line in on the mixer, but OTT for a mic in, and that I will get hum from the long lead if unbalanced."

Mixer line in levels can be quite sensitive if you turn up the input gain - on my Soundcrafts they are about 20dB lower gain than the mic input and can actually be used on high output mics like electrets (e.g. Microvox) so should have plenty of gain.

I got a very faint buzz on the mixer input, and found by experiment that it was lower if the ground side was tied to mixer ground even though it's a balanced input, but that was at the end of about 15m of unshielded twisted pair. With shielded cable you'll probably be fine unless there's a BIG transformer nearby.

It is, as you say, worth a try.

Anahata

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 05 Oct 10 - 02:07 PM

The run is about 30 feet - alongside speaker cable all the way, and past 4 sets of 220V sockets, but the ring main will be at right angles to the signal lead and the only other transformers will be in the mixing desk, the Pioneer, the CD player, the tuner, the two twin cassette decks and the old reel-to-reel.

Off to ebay to buy some twinplugthingies and some horrendously long phono leads.

Subject: RE: Twin output 'hifi' preamp
From: Bernard
Date: 06 Oct 10 - 03:39 PM

There are some nifty devices around these days that shove line level down CAT5 cables...

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 11 Oct 10 - 05:04 PM

Well, at present I am experimenting with stuffing nearly line level down thin and long single core screened lead. The kick drum has definitely come alive, vindicating both the preamp stage in teh Pioneer HiFi amp and teh power stages in teh Yamaha mixing desk, but I ahve a fair bit of hum on the right hand channel. Well, it's audible in the absence of signal at the equivalent settings for clipping on the Yamaha (that's 375+375 into 4 ohms) in a 30 foot drawing room, so it's quite loud.

Next possibility using mic leads with XLR-phono adaptors which might be better screened or maybe using the mono out on the Pioneer and taking all the top out on the mixing desk so it's only producing thud. The merit of that would be that the Celestion compression drivers on the PA speakers are not as sweet as the dome tweeters in the Castles on the HiFi speakers, and the downside would be that I could not get it as loud.

Subject: RE: Twin output 'hifi' preamp
From: Bernard
Date: 13 Oct 10 - 12:34 PM

Hum? Does it not know the words?!

Seriously, though, look for an earth loop (okay, I know you probably have!).

For people who don't know, an 'earth loop' exists typically where two pieces of kit are connected to different earthing points, and a small current is induced into the earth wire causing hum...

How to get rid of it and maintain safety is a rather big topic!!

Subject: RE: Twin output 'hifi' preamp
From: Richard Bridge
Date: 13 Oct 10 - 05:14 PM

I don't think it's loud enough to be an earth loop, and I bet that both earths are common at both ends. I might try turning some connections round tomorrow night.