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Old 26th May 2010, 06:33 PM   #21
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Slowmo View Post
I will be building a similar circuit as well so maybe I join this discussion.
Input impedance is set by Rin, but can you explain why it is necessary to lower the impedance of the opamp circuit? Is it because audio source needs to have some load in order to perform well? I will be tapping between DSP and amplifier chip to add line out so I guess I don't need Rin resistor, right?
Do you need Rin ? depends on the circuit as a whole.
If you mean the non inverting amp, Rin also sets a DC voltage (ground) on the input pin which is essential if the device is AC coupled.

On the inverting stage it just sets input resistance whether AC or DC coupled.

A high Rin gives total compatability with all sources. Remember some old DIN spec stuff might have something like 10 or 100 k output impedance. Loading that with 10k just kills all the signal.

Perhaps if you start a thread and post a circuit with what you are trying to do
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Old 26th May 2010, 08:07 PM   #22
ss007 is offline ss007  United States
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Great! thanks Mooly!

I'll digest the power supply situation. I'm trying to take enough time to understand what I'm doing here. I want to really learn as I go. You have been an incredible help so far!

OK, so I'll be using a dual power supply to power an OPA2604 [ in took a peak at some of the opamp discussions ] as you would.

Can you point me towards the simplest circuit to use?

I do want to use a pot. Would the pot take the place of R1 or would it just be an attenuator after the gain stage?
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Old 26th May 2010, 09:08 PM   #23
ss007 is offline ss007  United States
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Going back through the thread. I'm thinking the first circuit you posted is all I'll need.

What I don't understand is how the +/- voltage ties in.
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Old 27th May 2010, 06:34 AM   #24
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by limelight View Post
Going back through the thread. I'm thinking the first circuit you posted is all I'll need.

What I don't understand is how the +/- voltage ties in.

The Gainclone uses a split supply.

The split supply means the output from the amp is normally zero volts (at ground potential) but can swing either positive or negative with respect to ground.

You have to understand that concept... the amp can push the speaker cone "out" by altering it's voltage closer to say the positive supply and pull the cone in by going toward the negative supply.
At all times the centre point or "zero" of the supply is our reference that all measurements are made from.

The opamps normally run on a similar split supply. The quiescent state is "zero" volts at the output, but they too can swing both positive and negative.

Nelsons B1 uses a single supply, ground and +24 volts. So keeping to our reference point (ground) the output can only ever go more positive (toward 24 volts). It can never deliver the "negative" part of the signal.

This is the bit you must understand,
So what we do is bias the output of the B1 (or an opamp that's using only single supplies) so that the output is sat at one half of the supply.
So again in Nelsons amp the output will be at +12 volts DC.

We MUST AC couple the amp using caps at the input and the output.

Any signal applied is now handled by the circuit, with the output swinging either higher than +12 volts or lower than plus 12 volts within the limits og the 24 volt supply.
To enable us to use that signal and couple it back to a DC coupled design like the gainclone means that steady bias of +12 volts has to be removed, that's what the output cap does on the B1.

So the simplest circuit to use... You must decide how you are going to power it.
1. Are you going to run it from the single supply to the B1.
2. Or arrange a split supply from the Gainclone.
3. Another option is to use the split supply of the phono preamp to power it.


Do you want the amp in one box ?????? in other words an integrated amp, or do you want everything in it's own box, all separates, each with it's own PSU.

Personally I would go for making an integrated, or at the very least powering all circuits from the one power amp. There's nothing worse than loads of PSU's and leads etc.
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Old 27th May 2010, 06:37 AM   #25
Slowmo is offline Slowmo  Latvia
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Quote:
Originally Posted by limelight View Post
What I don't understand is how the +/- voltage ties in.
There are two pins for that which are left out in the circuit.
By the way, most often I see two opamps in a cascade. Is it because lower gain on each individual opamp is better than having higher gain on just one opamp?

Last edited by Slowmo; 27th May 2010 at 06:41 AM.
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Old 27th May 2010, 06:57 AM   #26
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Slowmo View Post
There are two pins for that which are left out in the circuit.
By the way, most often I see two opamps in a cascade. Is it because lower gain on each individual opamp is better than having higher gain on just one opamp?
Sometimes, it depends on the circuit. At high gain the HF performance of the opamp deteriorates, so using more than one device is one option, cheaper and easier than designing a discrete solution or using a dedicated higher performance part, that might be less suitable in other areas.
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Old 27th May 2010, 07:30 AM   #27
Mooly is offline Mooly  United Kingdom
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I would thoroughly recommend a read at this,
The Audio Power Amplifier Design Handbook
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Old 27th May 2010, 08:22 AM   #28
ss007 is offline ss007  United States
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Thanks for the incredible explanation!

I definitely want everything in one box. Gainclone, B1, phono, and ipod opamp gain circuit.

Can the one +25 -25 transformer handle all that without compromise?
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Old 27th May 2010, 11:19 AM   #29
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by limelight View Post
Thanks for the incredible explanation!

I definitely want everything in one box. Gainclone, B1, phono, and ipod opamp gain circuit.

Can the one +25 -25 transformer handle all that without compromise?
It can easily handle it... you make sure there is no compromise by getting the grounding correct. And the rest of it

You have to have some idea how you are going to lay it all out... and it's vital that the main ground in the PSU is correct.
That means running a short wire from the connection between the two main reservoir caps to something (a bit of PCB etc) that you can make multiple connections too. That is the main ground reference.

I think you need to build it up one section at a time. Get the power amps working, initially with just a volume control at their input. Make sure they are OK and noise/hum free, then move on to the preamp stages.
Lay it all out on paper first, keep the transformer away from the input/preamp side of things physically.
One step at a time.
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Old 27th May 2010, 11:43 AM   #30
AndrewT is offline AndrewT  Scotland
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Hi,
Mooly has made a good job of guiding you through this basic electronics interconnection short course.

I would like to add, that you have two main choices when locating the various components.

1.) Integrated amplifier. This has the input sockets and Power Supply and pre-amp with gain and attenuator and Buffer and two Power Amps all inside one case. This requires that all your sources can drive the interconnect cables and the input of the integrated amplifier. The Buffer is not really required, it's an added extra that provides little if any benefit. However, if you decide to go with an inverting Power amplifier then a Buffer will ensure all the sources can drive the lowish impedance of the inverting inputs.

2.) Separate pre-amp and power amps.
The Power amp is just that. Input sockets, PSU and one or two amplifiers inside a case. And no attenuator, although switched gain settings can be an advantage.
The pre-amp has input sockets and PSU and pre-amp and attenuator and Buffer. This allows the preamp to be located beside the various sources or farther away. It also allows the power amp to be located close or much further away from the pre-amp.

This capability to locate all the major components some distance from each other is the main advantage of the pre+power combination.
I like my Power Amp right next to the speaker terminals
I like the Sources where it is convenient to operate/use them. Like the VCR is never under the TV.
I like the Attenuator close by the seating position.

How you want to operate the "system" determines which assembly route you take.
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Last edited by AndrewT; 27th May 2010 at 11:45 AM.
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