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Old 12th July 2013, 01:26 PM   #11
thommy is offline thommy  United Kingdom
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Quote:
Originally Posted by qusp View Post
what is the DAC?
It's weiliang's latest 9018 board with the 4 opamps and 80 MHz clock
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Old 12th July 2013, 02:21 PM   #12
M Gregg is offline M Gregg  United Kingdom
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Why don't you,

Just build a passive preamp if you have enough gain on the power amps?

i.e. just a dual gang volume control and an input selector.

If you are using long input cables to the power amps then you might need a driver section.
Normally the OP from a DAC will drive power amps to full volume so do you need any gain?

The term PRE-amp is misleading it could just be a passive device with no power connected.
If there are tone controls then there could be a section that puts back what is lost across the tone controls.
Some time ago a friend had a quite expensive preamp with gain stages etc. He asked if he could try my passive pre.
The passive pre was better. So horses for courses etc.

The other thing a Preamp might have is RIAA for records or NAB for tape equalisation.

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M. Gregg
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Old 12th July 2013, 02:40 PM   #13
thommy is offline thommy  United Kingdom
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Ok so preamp isn't a circuit, so much as the term for the device that collates all the different sources and presents them for selection, making sure that each one is giving the amp what it needs.

So all I need is input selection and volume control, providing the input impedance on the amps doesn't prove to be too low.
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Old 12th July 2013, 02:43 PM   #14
M Gregg is offline M Gregg  United Kingdom
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Correct,

Try just wiring a standard LOG pot of about 100K between the output of the DAC and the input of the Amps and see if it works.

Remember to connect the DAC output across the pot and the wiper to the amps.

This link is quite good.
http://sound.westhost.com/pots.htm

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M. Gregg
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Old 12th July 2013, 02:46 PM   #15
thommy is offline thommy  United Kingdom
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I think that I will buy the amps and power supply. I have a little tube pre here that I can use to see what the input gain is like.

What would be the result of the input impedance being as low as it is if the volume knob fully open did not provide enough gain?
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Old 12th July 2013, 02:48 PM   #16
thommy is offline thommy  United Kingdom
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Sorry that last bit doesn't make much sense.

What problem is caused by having an input impedance too low?
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Old 12th July 2013, 02:48 PM   #17
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by thommy View Post
I think that I will buy the amps and power supply. I have a little tube pre here that I can use to see what the input gain is like.

What would be the result of the input impedance being as low as it is if the volume knob fully open did not provide enough gain?
If the gain is to low the power amps will just not go loud enough.
They will be low volume.
The link in my last post is interesting.
You might get some high frequency roll off. But if you have a tube preamp if it has a driver stage then there is no problem.

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M. Gregg
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Old 12th July 2013, 02:53 PM   #18
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by thommy View Post
Sorry that last bit doesn't make much sense.

What problem is caused by having an input impedance too low?
If the input impedance is low then it might need a driver stage however I don't think the output from a DAC would be unable to drive it.
You might get some HF roll off but its got to be worth a try first.

Think of it like this, if you plug your IPOD HP input into the input of the amps it will drive it and you will be able to control the volume from the ipod.

So if you plug the output from the DAC into the power amps they will probably be flat out. Your speakers might get damaged. So you always test with some cheap speakers first and always at low volume, then turn it up.

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Old 12th July 2013, 02:56 PM   #19
GoatGuy is offline GoatGuy  United States
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Default What preamps do

Quote:
Originally Posted by thommy View Post
Ok so preamp isn't a circuit, so much as the term for the device that collates all the different sources and presents them for selection, making sure that each one is giving the amp what it needs.

So all I need is input selection and volume control, providing the input impedance on the amps doesn't prove to be too low.
"coll[ects] all the different sources"

Yes, maybe, and not quite! The functional components of a modern, competent multisource preamplifier are

● A selector (rotary, push-button, electronic, etc) switch

The selector will have specialized "function specific" channels along with quite general purpose ones (though they might carry inspired channel names such as VCR, DVD player, yada, yada)

The channels that are more specialized are:

● A specialized equalization circuit for a turntable-input, if present
● A specialized equalization circuit for a raw tape deck

The rest are pretty much bog standard. However, to account for relatively high impedance inputs (which haver very little power, and need to be sturdied up for the following attenuator), most of the input channels will pass through a non-special-equalized preamplifier first stage.

Now that the input's impedance has been dropped to the 1K to 10K range, the output of the first stages of preamplification may be routed through:

● Bass/Mid/Treble tone contouring controls
● Sub-bass reinforcement
● Dual-channel sub-bass side-channel extraction (low pass filtering)

Which often (but not necessarily) again raises the output impedance remarkably ... making the signal generally unsuitable for feeding to the power amplifier. So... there is the 'output stage' of the preamp:

● An output + attenuator stage

This typically is a 0 dB (unity gain) stage that is ultra-linear. The output impedance can be "600 ohms" which is typical for P.A. equipment, or a few K ohms for personal stereophile stuff. The output of this is the ideal spot to place the stepped (or just potentiometer-type) attenuator.

A 600 to 5,000 ohm unity-stage impedance is simplied tied to a 2.5K to 10K (net) attenuator, and tapped all the way down. If one remembers the Thevenin and Kirchoff theorems, then the output impedance will always be the parallel-resistance of the attenuation network and the output stage impedance.

Voila!

For your DAC, you quite probably could just use a stepped attenuator, directly, and without all the above. Its just a very specific solution to the more general situation.

GoatGuy
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Old 12th July 2013, 02:57 PM   #20
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I have used many pre-amps and passive pre-amps(some people do not like to call them passive pre-amp...lol) and ALWAYS the passive pre-amp won
When passive is used...We must use the shortest connection from sources to inputs as possible and that includes the speaker wires. all this is due to impedance and all that crap. Some people use to much math in this area and leave the listening test for last. Numbers can show perfect results, but your ears can say something different.
I am a "passive- preamp" fan!

PS: I do not use Phono!...since most of the sources (I have not found one that has too low outputs yet) have enough output for the gain of my amp.I can easily play any kind of music without any problems.
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