High-end preamp for my GC

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#441
I built another one of these today. Its meant to drive my Sennheiser HD650 headphones, and it´s running a OPA637AP + BUF634 at a gain of 11. Sounds good at first try. Will hopefully get some measurements next week.

In my preamp, I prefer to use just one single OPA627BP for each channel, and no BUF at all... but this is of course up to everyone to make their own choice.

Im now running the DIL version of BUF634, and it´s getting pretty hot. Maybe I´ll have to try the TO220 version later on..

🙂
 
#445
I haven't tested myself but you can get a very small reduction of distortion by using max current but I'm perfectly happy with "medium" speed which is 3.9 kohms from pin 1. In my case I gain so little when I listen (Carlos I do listen to my stuff!) so the only advantage is a hotter amp.... and I have limited resources in that particaluar design.

Schematic can be found here, here and here.

Distortion with medium speed is less than my sound card, 0.008%

In the QRV-04 design I have also a current source for the opamp but with OPA627 and AD8610 I can't pick any significant difference with and without class A biasing but I have circuit on the pcb as an option.
 
#446
Ooops.:clown:

I did miss the part in the datasheet, where it says that Wide BandWith mode sets Iq to 15mA. That explains the heat in my Buf´s 😉

Well, I´ll just have to change them to TO220 and use heatsinks, like Carlos suggests.
 
#447
Yeah, after completing the amp, i figured the DIP devices were unsuitable for headphone opperation, i burned myself on them. :xeye:

I'l work a litle more on the amp at school next week.
(used the buf634 schematic, not yours carlos. any big differences in sound you think?)
 
#448
peranders said:
Distortion with medium speed is less than my sound card, 0.008%
Click to expand...

May I ask what soundcard and software you are using, and how you perform the measurements?

I tried measuring with my crappy integreted soundcard using a software called "Spectralyzer" (demo). All I did find out, is that I should get a new sound card 🙂

At first I measured with the mic input grounded to recieve the noise from the soundcard itself.

The second measurement was with headphoneamp connected to mic input, and inputs to headphoneamp grounded. This showed almost exactly the same as the first measurement.

Third one, I connected the input of the headphoneamp to the output on the soundcard (playing an infnite zero track). Now I got some really ugly noise, even with the sound pot turned to zero.

Is this the way to measure, or how should I proceed? (probably with a better source of course)
 
#450
Hetsinks for DIL

air said:
Ooops.:clown:

I did miss the part in the datasheet, where it says that Wide BandWith mode sets Iq to 15mA. That explains the heat in my Buf´s 😉

Well, I´ll just have to change them to TO220 and use heatsinks, like Carlos suggests.
Click to expand...

Or use these:

An externally hosted image should be here but it was not working when we last tested it.

Search for 75-611-52 at ELFA

I use these on the BUF634 and AD811 in my preamp...
 
#452

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#455
peranders said:
My BUF634's get rather warm, not hot when they are soldered onto a groundplane pcb and biased to max current and +- 15 volts supply voltage.
Click to expand...

As you know, I use them in wide BW mode, with +/-18v PSU.
And I use the TO220 version.
They get hot without the heasinks, but no problem with any "normal", small heatsink like those we use for regulators.

Also, as I said some pages ago, the TO220 allows a much better layout.
Better PSU arrangement, bypass, smaller feedback loop.
 
#456
air said:
I have measured now, with my crappy soundcard. Tested both with Sennheiser HD650 (300ohm) and with my old Philips SBC3388 (34ohm).
http://people.arcada.fi/~nylundj/rmaa/headphoneamp.htm
Click to expand...
Yes, your soundcard isn't the best around these days. I wonder though why you have so much distortion, much more than I have with OPA637+BUF634 :scratch: Maybe the difference is the pcb?

Here is the test for my OPA627+BUF634
http://www.sjostromaudio.com/hifi_files/qrv/qrv04_testreport/qrv04_test.htm

The test is done with resistive load but unfortunately I don't know how much I used. I will make a more detailed measurement later.
 
#460
loong said:
Some one offer me a Aleph 1.7 clone at very attractive price. I wonder how / if it goes well with the snubberized GC (25W) monoblock amp?
Click to expand...

With an (unbalanced) gain of 6.3x, that pre is not a good partner for your amp.
That's too much gain. You could change it later, though.

Also, I don't like that "0~800 ohm" output impedance spec, but that's me...
 
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