Simplest Passive Preamp design

[kinku]

Hi,
I am looking for a simple pre amp/attenuator design to use between a DSP and power amp.
DSP(MiniDSP HD) out put is 2V RMS and the amplifier is Adcom GFA 545 II.
Purpose is to protect the amplifier and woofers as a second level of protection.

Is it possible to do this using a passive circuit?
Link to Minidsp HD
https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd
Link to adcom Gfa
http://www.adcom-usa.com/userguides/gfa-545II-ug.pdf
Thanks

 

[AndrewT]

A potentiometer makes a passive vol pot.
That's all you need if you keep the parasitic capacitances very low.
For volume control of a stereo source you need a log law (or audio law) twin track potentiometer.

 

[kinku]

Is it that simple? Wouldn't that cause impudence issues ?

 

[AndrewT]

You have not given any impedance data.
Does that mean you don't care what they do?

 

[kinku]

Andrew Thanks for helping out. this is not something I am familiar with. So my power amp has an input impedance 100,000ohms.
Adcom gfa 545 II is my amp. The input sensitivity is 1.25Vrms and for Minidsp out put is either 4Vrms or 2. The amp is going to drive 3Ohm load.
Not sure about Minidsp's output impedence.

 

[AndrewT]

Use the 2Vac option for the minidisp. That is already too loud for a 1.25Vac input sensitivity.

The 100k input impedance allows you to use ~40k vol pot. so either use 20k, or 50k. But you may get slightly better performance by using a 10k vol pot.

Can the sources drive the cables and the vol pot?

 

[DF96]

Purpose is to protect the amplifier and woofers as a second level of protection.

The main purpose of a 'passive preamp' is to adjust the volume.

A 2V signal going into a 1.25V input is unlikely to do any damage unless it is a continuous test tone or very modern music with no dynamic range.

 

[kinku]

so to give you a better idea
The chain looks like this
Logitech streamer -->toslink-->minidsp HD -->attenuator/ preamp--> adcom GFA545--->2.75 ohm woofers( two eminence Alpha 15 parallel )
So my source would be minidsp HD I think.

 

[kinku]

But you may get slightly better performance by using a 10k vol pot.

Can the sources drive the cables and the vol pot?

Did you mean 100K Andrew ?
Hi DF 96, I can use the digital gain in minidsp but prefer to use one in analogue domain

 

[AndrewT]

Did you mean 100K Andrew ?
Hi DF 96, I can use the digital gain in minidsp but prefer to use one in analogue domain

you may get slightly better performance by using a 10k vol pot.

I really did mean 10k vol pot.
You could even try a 5k or 2k or 1k vol pot if you find out what your source is capable of driving.

 

[kinku]

Andrew please explain a bit more about the source. Is it not minidsp my source?

 

[AndrewT]

the source has to drive the parasitic capacitances, the RF attenuation capacitance and the Load all in parallel.
You need to ensure there is sufficient current capability in the source.
Then you need to arrange for sufficient voltage transfer to the load. This is achieved by setting a source impedance : load impedance ratio of >>1:5

 

[kinku]

Andrew is it better to do active preamp which may not have matching issues?

 

[AndrewT]

If the source cannot drive the cable and the load, then attach a Buffer to the source's output.

 

[kinku]

If you can help me out by describing how to do these in me abstract ways. Like circuits components. Etc

 

[rayma]

Is it possible to do this using a passive circuit?

The DSP output impedance is 560R, and the amp input impedance is 100k.
Just the situation for a simple 10k volume control. Standard 1m audio cables
will be fine for both the input and output of the volume control.

 

[kinku]

Rayma thanks for input.is this output impedence is only for Minidsp HD or is standard through out DSPs.
Can you guys please post a diagram of somekind to show the wiring.

 

[rayma]

is this output impedence is only for Minidsp HD or is standard through out DSPs.

I doubt there's a "standard" Rout, but it should be low, say under 1k.

 

[Ludus Tonalis]

I'm sure that a 10k dual log pot in a small metal box will be fine. I have been using such a "preamplifier" for many years. I think you will be surprised at the sound quality of this simple arrangement. There's a sticky thread on the subject in this forum.

I don't have input sockets on the box; I have flying leads with phonos at one end for the CD player and XLRs at the other end for the power amplifiers (2 separate single channel amps). If you do it this way, use grommets to avoid any damage to the cables where they enter the box.

It's such a cheap option that it is worth trying before anything else. I would, however, recommend steering clear of the cheapest potentiometers. Get a Bourns or something similar.

I've attached a simple diagram. Attach all the wires that need to be grounded (the four shields and the two wires from the pot) to the same point on the box. Solder tags on a bolt through the box are a good way to do this.

Good luck!
Stephen

 

[kinku]

Thanks Lodus , that helps.do you happen to know the link to that sticky thread ?