(re)searching for a better preamp

jaudio · 2005-12-26 3:43 pm

One more picture

Thanks Carlos.

carlosfm · 2005-12-26 4:00 pm

Let is

-in for a few days and take a listen.
It will be hard to shut it down, when you start listening. :violin:

jaudio · 2005-12-26 4:55 pm

Im listening to the headphone amp driving small computer speakers and I can tell you the AD815 are driving the h*ll out of these speakers.

From 7ft away I can hear it very clearly. Now, think of what it can do for headphones.

carlosfm · 2005-12-26 9:36 pm

jaudio said:
Im listening to the headphone amp driving small computer speakers and I can tell you the AD815 are driving the h*ll out of these speakers.

neutron7 · 2005-12-27 6:37 am

jaudio did you just bend the legs of those ad815 a bit to make them fit the hole spacing of that board?

btw i hear brass doorknobs sound a lot better than frosted glass ones you should try it!

jaudio · 2005-12-27 10:50 am

neutron7 said:
jaudio did you just bend the legs of those ad815 a bit to make them fit the hole spacing of that board?

Yup I just bend them. If you bend them be careful.

neutron7 said:
btw i hear brass doorknobs sound a lot better than frosted glass ones you should try it!

Ha Ha It is there to hold the aluminum strip to the tab. I heard silverplated gold is even better. :clown:

idiotcountry2 · 2006-01-03 5:27 pm

Anyone have an opinion as to the benefit/detriment to putting the Pre-Amp on the same PCB or in the same enclosure as the power amp? Would seem to me idea for the shortest runs of both signal and power cables.

Thanks,
tom

carlosfm · 2006-01-03 10:09 pm

idiotcountry2 said:
Anyone have an opinion as to the benefit/detriment to putting the Pre-Amp on the same PCB or in the same enclosure as the power amp? Would seem to me idea for the shortest runs of both signal and power cables.

Thanks,
tom

No problem, as long as you use dedicated PSUs, and you can keep speaker cables fairly short, which is also important.

Panelhead · 2006-01-03 11:06 pm

Circuit in application notes

I ordered 10 of the SIP package ones a while back. Looking at the AD application notes, there is a differential line driver shown on page one.
My present linestage has input transformers and output transformers already. I looks like all I would have to do is switch to dual volume pots after the input tranny to use this circuit.
I most likely would scale the 499 and 110 ohm resistors to lower the gain. Unity gain works fine in my system, using some 200 ohm for all three looks like it should work okay. I would never notice a gain of two.
I know this is offthread from this fine circuit shown as a single ended preamp. But has anyone here (Carlos) tried this chip as a differential line driver?
I originally was going to build the circuit as described here, but seeing the line driver with no coupling caps got me thinking.

TIA,
George

carlosfm · 2006-01-04 12:04 am

Re: Circuit in application notes

Panelhead said:
I would never notice a gain of two.

Are you sure?
That would double the gain of your system, and that is very noticeable.

Panelhead said:
I know this is offthread from this fine circuit shown as a single ended preamp. But has anyone here (Carlos) tried this chip as a differential line driver?

No, I didn't try it as differential.

Panelhead said:
I originally was going to build the circuit as described here, but seeing the line driver with no coupling caps got me thinking.

I don't use the coupling caps on my preamp. I started using it, but right in the next day I tested direct output and never went back.
If you see my schematic, it has two outputs, one with and one without coupling caps. I have both outputs on my pre, but I'm using the direct output for connecting to my power amp. The other output (AC coupled) is connected to the sub.
The direct output has low DC offset, around 30~40mV, worst case, depending on the volume pot position, and I have input DC coupling caps on my power amp.
I don't recommend is using DC coupling caps on both the pre and the power amp.

I'm not sure if your results with input and output signal trafos will be better, but I doubt. You are inserting more (passive, and band limited) components on the input and output of the pre.
At least they should be good and relatively big, or they will saturate with the relatively high DC offset, if you don't thake similar measures as I did to minimize it.
Because to minimize DC offset you must use very low input resistors to ground, which will create a shunt and attenuate the signal. An input buffer would then be needed, and more gain on the AD815. Nah...

I think that between the compromises, and after several tests and putting my head to work, the solution I opted for is one of the (if not the) most transparent ways of using this current feedback video op-amp as an audio preamp.

idiotcountry2 · 2006-01-04 1:12 am

carlosfm said:
No problem, as long as you use dedicated PSUs, and you can keep speaker cables fairly short, which is also important.

Carlos,

Do you mean one power supply for the Pre-Amp, and one power supply per Amp channel - a total of three power supplies for a two-channel Stereo amp? Is it advisable with your design to then have three isolation transformers?

Sorry for so many questions!

tm

carlosfm · 2006-01-04 3:00 am

idiotcountry2 said:
Carlos,

Do you mean one power supply for the Pre-Amp, and one power supply per Amp channel - a total of three power supplies for a two-channel Stereo amp? Is it advisable with your design to then have three isolation transformers?

I mean at least give the pre a dedicated PSU, including trafo.
Even if you are building an integrated amp, treat it as independent devices, and don't use the same PSU of the power amp to feed the preamp seccion.
For those kind of compromises we already have commercial gear out there.

dhenryp · 2006-01-04 3:40 pm

I have question regarding the DC nulling circuit with its dedicated regulator; What is the reason that you could not derive the same thing from the AD815 positive supply? It would seem that you could get the same current with some changes to resistor values or are you using it simply as an extra level of regulation?

Thanks,

Denis

carlosfm · 2006-01-04 7:38 pm

dhenryp said:
...or are you using it simply as an extra level of regulation?

Exactly, that was the point.

idiotcountry2 · 2006-01-07 12:08 am

carlosfm said:
I mean at least give the pre a dedicated PSU, including trafo.
Even if you are building an integrated amp, treat it as independent devices, and don't use the same PSU of the power amp to feed the preamp seccion.
For those kind of compromises we already have commercial gear out there.

And just to drag this thread out even more...

What's the thought on a single transformer with dual secondaries for the power amp (stereo) and then to use an additional transformer with a single secondary for the pre-amp? Or to use one transformer - one secondary feeding the pre-amp and one secondary feeding the Power Amp?

I don't want to re-invent the wheel if someone has already done it

I would love to get away with having as few parts as possible and still have great sound.

t

Nordic · 2006-01-07 6:56 am

I think Carlos is right to a point here...
the other day, I decided to try out the little zener based regulator from the regulator thread, well, when I went to pay for the components, it cost almost as much as a small transformer!

rudi · 2006-01-10 5:28 pm

Finally a Pre amp that gets it right

AWESOME

I have just finished my version of Carlos's take on the AD815 (it uses a surface mount device) and i am stunned with the performance. Nice one Carlos, well done!

I am running it with my X-Calibre and the combination is simply mind blowing. For the first time my hifi disappeared and i had a FULL stage with lots of layering

I have tried the OPA 627 variations and frankly it is just a waste of time. This is the one to use

rudi · 2006-01-10 5:31 pm

just to add

I am using a CS3310 ( you can substitute it with a BB PGA2311 if you want better spec) as a volume control and this is also used in the original Jeff Rowland coherence pre.

you can get a very nice pre module from these guys in Hong Kong
http://eshop.diyclub.biz/product_info.php?cPath=85_187&products_id=196

carlosfm · 2006-01-10 7:36 pm

rudi said:
AWESOME

Russ White · 2006-01-10 10:08 pm

rudi said:
just to add

I am using a CS3310

Hmmm the CS3310 has an opamp which is actually much worse than an OPA627 on its output. So if you are feeding your new pre with and inferior opamp... whats the point?

I have tried the AD815 some time ago (back when this thread was young), and would agree it is top notch.

Not sure I would go spasmodic with ecstasy, but very very good none the less.

Cheers!
Russ

(re)searching for a better preamp

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