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Help with a pre-amp

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2009 at 3:49am
Andy,

It's been done with transformers. The only difficulty is that transformers do it by impedance ratio, which means you can only have voltage "gain" with current "loss".

Take the phono cartridge step-up transformer as example: its input is low impedance (high current) and low voltage to match a moving coil, and its output is high impedance (low current) and higher voltage to match a moving magnet.

Somebody told me about a transformer "preamp" which had input impedances of 100k and a gain of two...

A 200k output impedance isn't much good at driving a 22k input impedance power amp...

It gives you a net loss - never heard of that "preamp" since.

As transformers are taught at technical college at HNC level you'd have thought the penny would have dropped with this particular designer? Obviously it didn't as he was no engineer, period.

A balanced output has one input and two outputs, one inverted in phase by 180 degrees to the other - it can only be done using a real amplification device such as a valve or transistor. It can only have gain if done with more that one valve or transistor (and associated support circuitry which has to include a power supply).

Actually, thinking about the above, it can be done passively with capacitors and resistors but only at one narrow band of frequencies, and would have such an insertion loss as to be completely useless.

Just in case some think it, the above is in no way Graham Slee guile - it is a bit of scientific fact.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy9672 Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug 2009 at 9:03am
Amazing the number of companies that put products onto the market without knowing how they work! 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iamalexis Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2009 at 1:22am
Originally posted by Graham Slee Graham Slee wrote:


Just designing one hopefully to launch in the Autumn partly in response to this topic.I've been often asked for a box that does "single-sided" domestic in to balanced out.Or in this case a volume control with same.And often requested to simply do a two or three input preamp.


Graham I'm intrigued to know more about this, are there any more details you can share with us    
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2009 at 12:56am
OK.

In a simple analogue signal system (one that doesn't need digital to analogue conversion because other equipment does it) where there are few sources, such as CD, Phono Amp, Computer/iPod to name three, such users would only really need three inputs.

As for a recording output, why? I remember recording cassettes in the dark ages for the car. If you want to copy a CD you'd simply pop it into the DVD RW tray on the computer and burn a copy. If it's on computer you just duplicate it on the computer. If it's vinyl then maybe, but a split feed - one to control preamp and one to computer will suffice. So do we really need rec-out?

What most power amps want is drive voltage. A passive pre can't add voltage, it can only lose it.

Then there are active loudspeakers - loudspeakers with amps built in - not the cheap computer things with 10 zillion watts pmpo that wouldn't tickle a gnats ch.... , but grown-up types.

There always seems the need for balanced outs even if it's just to boast to the mates...

And then there are proper uses for balanced outs such as driving a long cable.

Or for professional users who use balanced lines to avoid ground loops.

But a full blown pre which does music well costs £?,???.??, which is OK if you can afford to blow that amount for a simple analogue system, but a £250 little box solution can sound as good if not...

If possible I would fit it in a case the size of the Novo because we can get them at a good price in bulk, and they look good. It would not take up valuable hi-fi real-estate.

However, it does mean hefty interconnect cables could cause it to do a wheelie, but at that price blu-tack wouldn't be objectionable???

So round the back the usual 4 way phono block we use for two inputs, and a 3.5 jack on the front for the computer feed - howzat?

If OK, then 2 x XLRs on the back if they'd fit. The other way of doing balanced is two TRS jacks - one per channel (a normal 1/4 inch stereo jack socket) wired phase=tip, antiphase=ring, ground=sleeve. That's what "busy" mixers use for balanced ins/outs, such as the rack-width Soundcraft's. On ocassion the serious pro-user will want B-guage (PO) plug compatibility and that can be accomodated.

So what else?

No remote control - we need the exercise - and some would get used on the desktop anyway.

Input switching using a simple manual rotary "dry-contact" (Gold plated contacts) 3-way switch.

Volume.

Balance.

No room for tone controls on this one, but use one of our phono preamps - they sound good...

Perhaps a very simple headphone output???

And there you have it.

OK, forgot the power supply. PSU1 or wall-wart. The "power share" thing I'm putting into action allows anything we sell with the power share feature (to be announced elsewhere) to share its power supply with another power share item.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrarroyo Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2009 at 2:48pm

A headphone out would be good if it sounds as good as the Novo's. Otherwise it may take up valuable real state in such an small enclosure.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy9672 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2009 at 6:59pm
Making the pre using phono blocks with the option of having a xlr/po plug at extra cost?

to have all wire comectors on the back, i like to have a neat front to my equipment!!

Also you could make a simple tone control using your 3 way switch, one for trebel one for bass with a 3db add or cut?

i would leave the headphone out but build in provision for two pre outs, one for the power amp one for the headphone amp

and all of a sudden your fitting a huge amount of connectors onto the back of a very small box!!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2009 at 12:07am
Having listened to what you say and having surveyed the rear panel of the Novo sized case, I can provide plenty of ins and outs round the back provided that a compromise can be tolerated?

I can provide phono sockets (RCA jacks) on two inputs (that's four sockets/two pairs like the back of the Novo/Solo/etc)

Balanced outputs not on XLR's but the alternative: TRS jacks like on mixers (1/4 inch stereo jacks wired differently)

And now the compromises...

A third input on a 180 degree DIN socket (like Naim use/used)

An aux in/aux out (pre-out) on a 240 degree DIN socket.

So, that's really 4 stereo ins and 2 stereo outs (one being balanced on left and right TRS jacks). And no wires into the front.

Acceptable? At the price?

As for tone, I could try a similar "slope" switch: up-position favouring the highs; down-position favouring the lows; and mid-position flat.

Input switching to use a similar 3 position switch controlling relays to minimise cross-talk -- that would fit.

Yet another 3 position switch selecting aux-out / nothing / aux-in (the nothing position being similar to the non-shorting Solo "mute")

And a volume control.

Are we there yet?

Or will we need a balance control?

PS. I thought of calling it the Aria


Edited by Graham Slee - 19 Aug 2009 at 12:14am
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