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Old 6th Feb 2020, 7:09 pm   #1
bikerhifinut
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Default Why use a preamp?

I recently converted a Kit valve amplifier from a Power Amplifier to its Integrated version due to winning the hardware needed cheap on the Auction site. It basically consists of an extra set of switched inputs, I used relay switching at the input sockets as it makes for a neater wiring job as much as anything.
The input is taken directly to a potentiometer with the wiper connected to the grid of the ECC83 input valve. It's a fairly standard 3 stage job with Long tailed pair phase splitter into push pull KT88 outputs for 35 watts (measured by me on the scope).
Sensitivity is 450mV for 35W which is fine for all of my sources, I like to use as much of the volume control travel as possible rather than the first 10 degrees of rotation.
Anyway I think it sounds just fine and dandy without the preamp in circuit, I was using a cathode follower buffer stage with unity gain as previously in another room I had the power amps at a distance from the rest of the kit and closer to the speakers. so a low impedance drive to the power amp made a bit of sense with a long-ish connecting cable.
So my reasoning was this. If the preamp works by having the wiper of the pot straight onto the grid of the input valve with a 2 inch length of wire and there's obviously no degradation of signal, then surely the same thing at the input of the power amp is the same? i.e. no adverse electrical effects on the audio signal.
I'm not looking for a long technical explanation but I think I have understood the only real justification for a preamp/buffer stage when no extra gain is required?
Thanks for any input.

Andy
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Old 6th Feb 2020, 7:18 pm   #2
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Default Re: Why use a preamp?

The usual reason is - for vinyl-record-playing types - that the preamp incorporates frequency-range-levelling/equalisation.

https://en.wikipedia.org/wiki/RIAA_equalization
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Old 6th Feb 2020, 8:02 pm   #3
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Default Re: Why use a preamp?

Quote:
Originally Posted by bikerhifinut View Post
... I think I have understood the only real justification for a preamp/buffer stage when no extra gain is required?
I think you have too. As long as it can be placed close to the power amp and as long as you don't need the RIAA correction that G6Tanuki refers to then the so-called 'pot in a box' solution can work fine.

Of course back in the day 'real' pre-amps would include tone controls and filters. These are inevitably lossy, so at least one active gain stage would be required to correct for that.

I have seen expensive modern pre-amps with several valves glowing conspicuously and prettily on display and, well-hidden in the background, a resistive divider giving a healthy dose of attenuation so that the volume control still has a useful range of operation . Noise introducers. But if they make folks happy ...


Cheers,

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Old 6th Feb 2020, 9:40 pm   #4
bikerhifinut
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Default Re: Why use a preamp?

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Originally Posted by G6Tanuki View Post
The usual reason is - for vinyl-record-playing types - that the preamp incorporates frequency-range-levelling/equalisation.

https://en.wikipedia.org/wiki/RIAA_equalization
Yes I get that, sorry I should have made myself more clear. I use a separate RIAA preamp in most situations, and separating the very low level and noise sensitive circuitry from the power amplifier is effective.
A.
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Old 6th Feb 2020, 10:08 pm   #5
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Default Re: Why use a preamp?

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Quote:
Originally Posted by bikerhifinut View Post
... I think I have understood the only real justification for a preamp/buffer stage when no extra gain is required?
I think you have too. As long as it can be placed close to the power amp and as long as you don't need the RIAA correction that G6Tanuki refers to then the so-called 'pot in a box' solution can work fine.

Of course back in the day 'real' pre-amps would include tone controls and filters. These are inevitably lossy, so at least one active gain stage would be required to correct for that.

I have seen expensive modern pre-amps with several valves glowing conspicuously and prettily on display and, well-hidden in the background, a resistive divider giving a healthy dose of attenuation so that the volume control still has a useful range of operation . Noise introducers. But if they make folks happy ...


Cheers,

GJ
Ta Graeme,

It's, for me at any rate, an interesting practical exercise and I'll still be using an active preamp when I get my Quad 405 monos fettled up and back in the system. They'll be hidden behind the ESLs with a longish connecting cable from the rest of the electronics but about 6 inches of wire between their outputs and the speakers. No intentions of expensive and dubious Boutique wires, it'll be decent and inexpensive pro grade screened microphone cable. driven from a very low Z output on the preamp they are as good as it needs to get, I have over the years spent money on upmarket interconnects, never silly money but all the same...…… And now I make my own using good quality pro audio parts and we all know the difference...………………
But I'm quite pleased that I now don't need another box on the shelf with this amp. It's got me looking hard at my totally homebrewed EL34 power amp and how best to fit a nice panel on the front of the chassis and incorporate a couple of inputs and a volume pot! Again I know what you mean about a useful range of operation on a volume control which is why I think 500mV isn't such a bad sensitivity given the output voltages of most modern Digital sources and even my Phono preamp puts out a Volt so its nicely matched in.
Andy.
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Old 7th Feb 2020, 12:54 am   #6
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Default Re: Why use a preamp?

Why is it called a "preamp" when its just a pot? isnt it a "noamp" ?.
I have seen some audiophool units with a very flash box containing a dual pot ( Alps) and some "heavily gold plated " input and output sockets. They add colour and texture and warmth. Go figure

Joe
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Old 7th Feb 2020, 1:03 am   #7
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Default Re: Why use a preamp?

Sometimes called a passive preamp. Goes in before the amplifier and does nothing.
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Old 7th Feb 2020, 8:23 am   #8
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Default Re: Why use a preamp?

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No intentions of expensive and dubious Boutique wires, it'll be decent and inexpensive pro grade screened microphone cable. driven from a very low Z output on the preamp
Andy.
Just a small achtung!

Be careful when trying to engineer a low output impedance stage for driving cable. The capacitive load they present at high frequencies can lead to instability of the driving stage and oscillation at radio frequencies.

I use professional microphone cable from my preamp to my power amp. The cable impedance measured as 600 Ohms (it's unusual tinsel stuff) so the preamp uses NE5534 devices with feedback to make a low Z output, driving the cable through a 600 Ohm resistor (2x1.2k in parallel) The power amp was designed to present a 600Ohm load.

Given the cable length in lambdas, and the limits on my hearing, this is unnecessarily pernickety by orders of magnitude, but I thought that as I was doing it, I might at least do it properly. It serves to illustrate that low impedance output from a preamp isn't really needed.

Getting any amplifier to be stable into a load that looks capacitive at high frequencies is a well known design problem.

David
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Old 7th Feb 2020, 12:32 pm   #9
bikerhifinut
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Default Re: Why use a preamp?

Jawohl!
No worries David and as the preamp for the Quads is most likely going to be my trusty Rega Cursa3 which is well capable of driving long cables I feel absolutely confident.

Andy
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Old 8th Feb 2020, 2:04 pm   #10
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Default Re: Why use a preamp?

Quote:
Originally Posted by AC/HL View Post
Sometimes called a passive preamp. Goes in before the amplifier and does nothing.
Which, of course is exactly what you want it to do. Attenuate the incoming signal without adding or subtracting anything from it.
Unless you need or want tone controls of course and I'm not against them at all as if you have a device fitted with them and they are properly designed so as to have no effect when set "flat" then you do have the choice.
I'm not against any approach on religious grounds! In my book there's no "right" or "wrong", within reason.

Cheers.

Andy
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Old 8th Feb 2020, 2:27 pm   #11
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Default Re: Why use a preamp?

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Quote:
Originally Posted by bikerhifinut View Post
No intentions of expensive and dubious Boutique wires, it'll be decent and inexpensive pro grade screened microphone cable. driven from a very low Z output on the preamp
Andy.
Just a small achtung!

Be careful when trying to engineer a low output impedance stage for driving cable. The capacitive load they present at high frequencies can lead to instability of the driving stage and oscillation at radio frequencies.

I use professional microphone cable from my preamp to my power amp. The cable impedance measured as 600 Ohms (it's unusual tinsel stuff) so the preamp uses NE5534 devices with feedback to make a low Z output, driving the cable through a 600 Ohm resistor (2x1.2k in parallel) The power amp was designed to present a 600Ohm load.

Given the cable length in lambdas, and the limits on my hearing, this is unnecessarily pernickety by orders of magnitude, but I thought that as I was doing it, I might at least do it properly. It serves to illustrate that low impedance output from a preamp isn't really needed.

Getting any amplifier to be stable into a load that looks capacitive at high frequencies is a well known design problem.

David
Valid points especially if homebrewing a preamp or buffer circuit.

How does that affect something like a preamplifier used to drive a longer cable with a transformer output? Would the transformer be less prone to cable capacity issues?

And at what point does a cable become long? I won't be using anything longer than 3m and hopefully nearer to 2m which I think isn't really "long".

A.
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Old 8th Feb 2020, 2:49 pm   #12
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Default Re: Why use a preamp?

In the olden days when I started work, the preamp contained the various inputs, tone controls and equalisation for different inputs. Magnetic cartridges had a very low output and so needed the extra gain.

If you have sufficient gain on your main amp and don't need any frequency correction - you don't need one.

Peter
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Old 8th Feb 2020, 2:58 pm   #13
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Default Re: Why use a preamp?

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... It serves to illustrate that low impedance output from a preamp isn't really needed ...
You can get away with it as long as it's reasonable. I would call 600ohms low, as long as you're not driving tens of nanofarads of cable capacitance (multiple amps, each with a 20m cable, say). Adding a 600ohm output resistor costs you 6dB of signal. Is that really needed ? The extra 6dB of gain needed to make up for it will come with some amount of noise which we'd be better off without.

Cheers,

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Old 8th Feb 2020, 4:41 pm   #14
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Default Re: Why use a preamp?

Actually you can drive tens of nanofarads of cable. If the source impedance and the load impedance equal the cable's characteristic impedance the effects of the cable's distributed capacitance and distributed inductance cancel (in both directions!) So the preamp and its output resistor never see the whole capacitance of the cable or even a small part of it. A pure resistive impedance is presented to the preamp.

For RF, this is an important trick. For audio, it isn't really necessary, but if you can, what the hell, why not?

For a unidirectional signal flow with a well matched (non-reflective) load, then the source series resistor isn't necessary, but if there are imperfections, then it protects the preamp from seeing capacitive loads at HF, which could introduce an unplanned pole into the feedback loop and ruin its stability at HF.

There is 6dB signal loss, but with reasonable signal levels and resistors without gross problems with excess noise, there won't be a noticeable noise disadvantage.

There are other ways of moving signals around, but properly terminated transmission lines do the job and have a certain elegance.

David
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Old 8th Feb 2020, 5:21 pm   #15
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Default Re: Why use a preamp?

Yes, of course you're right. A matched-termination line looks like a resistor and provided the source can drive a resistance that low there will be no HF roll-off. I'm too used to valve gear where simple common-cathode stages struggle with 600ohms.

Cheers,

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Old 8th Feb 2020, 8:35 pm   #16
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Default Re: Why use a preamp?

2 or 3 metres of cable isn't "long" in this context.

Something like 100 metres is what I would call a long cable (and it's still nowhere near long enough to treat as a transmission line). Then the problem is not the filtering effect of the capacitance and the load termination. It's the ability of the source to provide enough current to charge the cable capacitance at high frequencies.
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Old 8th Feb 2020, 9:07 pm   #17
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... It's the ability of the source to provide enough current to charge the cable capacitance at high frequencies.
This was the mistake I made earlier. It's actually the ability of the source to provide enough current to drive the load resistance. The definition of a 'short' cable at any given frequency (wavelength) is one whose characteristic impedance (resistance) is much less than the reactance of its capacitance.

Cheers,

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