Preamp beginner questions, any help appreciated

[gaplessophile]

Hi, I'm new to chipamps and plan on building one. I'm interested in building a preamp to go with it. I've read posts on this forum and some information on other web sites, and just have a couple of questions about preamps:

Is it okay to have a preamp running off the same power supply as the chipamp, provided the voltage is okay? I'm sure this isn't ideal, but is it any worse than having both channels of the chip amp running off the same power supply?

I've seen many designs for preamps that include buffers, and some that are only buffers, without an opamp. Why is this? On a design that I'm looking at, for a chipamp, its input impedance is 20kOhms, which I would have thought wouldn't draw enough current from the source for buffers to be required. Could someone please explain?

Thanks for any help you can give me!

[Arius]

Brief preamp lesson. A preamp serves the following purposes:

1) Level/Gain matching. Your power amp may have an input sensitivity of 2V for full output. Your signal source may only produce 1V or less.

2) Impedance matching. It's not just about current. Your signal source may have an output impedance of 20K. Your power amp may have an input impedance of 20K. However at low frequencies, the input coupling cap will have a significant impedance that will drop the input level (bass will sound weak). Another reason you want a preamp to have low output impedance is to drive long cable lengths.

3) Volume and input selection.

4) Misc functions, e.g. single ended to balance, tape/record loops, tone controls.


In your case, your application is simple. For a preamp, you'd really need just a pot for volume control. Next is do you need gain? If you're trying to get a lot of power out of the chipamp, then you'd need more gain for a given input level. If that exceeds the gainbandwidth product of the chipamp, you'd need external gain (e.g. opamp). Next, if your source has low output impedance, you don't need a buffer. In most cases for chipamps, you can get by with a passive preamp - i.e. pot for volume, switch for input select.

[gaplessophile]

Great, thank you, that was very helpful!

Now does anyone have an answer to my power supply question? Is it okay for the preamp and chipamp to share a power supply?

[Russ White]

Originally posted by gaplessophile
Great, thank you, that was very helpful!

Now does anyone have an answer to my power supply question? Is it okay for the preamp and chipamp to share a power supply?

Pramps usually run with lower rails than most chipamps, so you would need to regulate the rails down to where the pre wants them, but there is no reason one could not share a PS, you will just need to make sure your preamp is designed(or modified) to allow for that.

Cheers!
Russ

[rabbitz]

Just use a pre regulator circuit like the P102 that comes after the chipamp supply and build your ±15V supply after that.

If the voltage of the chipamp is run off a 18V transformer, then you can pick up off the ±25V supply to feed the ±15V supply.

[gaplessophile]

Thanks very much to everyone who replied!

Another little question, I'm sure it's really obvious, but when you say ±25V, does that mean that the voltage rails are 25V either side of ground, ie +25 AND -25? Or does it mean that the voltage rails are 25V apart? And would a transformer with dual secondaries, labelled "18 + 18 Volts" be right to, when rectified, produce the ±25V I've seen recommended for a chipamp?

Thank you again for helping a newbie out!

[AndrewT]

Hi,
the 18 - 0 - 18Vac should give about +-26Vdc when unloaded and drop to about +-25Vdc when biased (if the chipamp runs much bias - it probably does not to keep it a bit cooler).
Yes, that is +25Vdc and -25Vdc relative to ground.

I would run dual regulators off the main amp DC rails.
regulate down to +-20Vdc inside the power amp case and use a 3pole socket and plug to send this DC to the preamp case.

Regulate to +-15Vdc inside the preamp to give a quiet supply to the preamp circuits.

I don't know if it makes a difference, but I would use different types of regulators for the pre-regulation from the final regulation. So that they have a different noise spectra and attenuate the high frequency rubbish (I do hate this forums auto delete for words it does not like) that otherwise may try to pass straight through.

The preamp will not need as much smoothing as the chipamp, I normally recommend 2mF/Apk (=2uF/mApk)of output current. Consider the possibility of using NO electrolytics in the preamp PSU nor in it's circuit. It will cost a little more but the benefits may transform your sound.

[gaplessophile]

Thanks for all that info AndrewT. I was actually planning on putting the preamp in the same case as the chipamp. Would this be a bad thing to do? If I was doing that, I guess I wouldn't need the two stages of voltage regulation?

[AndrewT]

Hi,
the chipamp demands high currents from the PSU and the resulting low and high frequency modulations of the supply rails will be quite severe.

I would still put in two stage regulation to help attenuate this supply variation from the pre-amp.

Another thought;- how about series regulator for first stage and low noise high speed shunt regulator for second stage?

[Arius]

Um, multistage regulation? Shunt regulators? No electrolytics in PSU? All these for a GC? More importantly, all these for a beginner?

It's all good but it sounds like an overkill for the thread starter's intend application.

Still, it's good info, I guess. If anyone wants to go ahead with Andrew's suggestions, it might be better to do so with separate mains transformer(s) for the preamp.

If Gapple still wants to put the preamp into the same case as the poweramp (i.e. integrated GC), if simplicity was the key, I'd do this: Main power rails ---> RC---> +/-15V regulators. I did that for my integrated GC and I have no hiss/hum (none!) at max volume. Grounding for integrated amps is a big deal and if you're new, you might encounter grounding issues with complex circuits.

Don't want to scare beginners away: The short story is: start small, then experiment around.