High-end preamp for my GC

[carlosfm]

Ok, here they are, the small boards.
It could be one big board with everything, but as I was doing it the ideas kept coming...

From left to right:

PSU
Headphone amp (OPA2132 / BUF634)
Preamp module (OPA627 / BUF634)
Buffer for the tape loops (OPA627)

I have an Alps Blue 20K log. pot, it's not in the pictures.
All this has been playing on the floor, because the box hasn't arrived yet.
The sound is fantastic, even my headphones kick an impressive sound.
I'll put some pictures of the finished Pre later.
Here it goes, general view:

 

[carlosfm]

A closer look at the pictures.
Sorry for the lowsy quality, but this second camera I have is a point-and-shoot autofocus and it has no macro ability.
I always get much better photos with my "Rolls Royce" Canon T90 manual focus.
A closer (not much...) look at the Pre and tape loop buffer.

 

[carlosfm]

Finally, a closer look (not much...) of the PSU and the Headamp module.
Is this page getting too "heavy"?

 

[Raka]

Heavy? Not at all, I listened to Metallica St. Anger yesterday, and this is heavy
I'm happy to hear that it sounds very well (pun?!), Does it mate well with your GC?
Which schemes were you using?
I really think that together with your planned small GC, you will have a minnuscule great gear!! Which case are you using?

 

[carlosfm]

Hi Raka,
Glad to see you listen to all kinds of music.
The preamp mates very well with my GC indeed, it's impressive.
What a shame I can't "send" you the sound...
I based on the headphone amp that's on the BUF634 datasheed, with some changes.
I normally don't do schematics, it's all in my head.
The case for this preamp is 26x6.5x18 cm (WxHxD), and it will be full of RCAs , 6 line-ins, 3 tape loops and 2 pre outs.
Ha!

 

[PWatts]

Interesting. I've built a headphone amp using an OPA2132 with a BUF634 as explained in the BUF634 datasheet and, no matter how well regulated my supply, short & shielded the connections and decoupled the opamp, there always was excessive noise. I finally gave up and built a discrete headphone amp.

 

[carlosfm]

Huh?

Interesting. I've built a headphone amp using an OPA2132 with a BUF634 as explained in the BUF634 datasheet and, no matter how well regulated my supply, short & shielded the connections and decoupled the opamp, there always was excessive noise. I finally gave up and built a discrete headphone amp.

ABSOLUTELY NO NOISE!
Did you use the BUF634 inside the feedback loop?
All the caps you see in my boards are there for PSU bypassing, no caps in the signal.
0.2~0.3 mv DC offset.
Op-amps were hand-selected.

 

[PWatts]

Yes. I experimented a bit with the BUF outside the feedback but it (naturally) was noisy as hell. I've decoupled both the opamp & BUF's supply rails with electro's in parallel with ceramics at less than 2mm from the pins. Maybe my cheap junk closedback ultra-sensitive 18ohm headphones made it more obvious. The noise was much less on a set of 30year old Sennheiser openback insensitive headphones. I always use my junk headphones for testing headphone amps because it's very easy to spot noise with it.
Pierre

 

[Raka]

Well, I like very much almost all kind of music, but when I say music I don't mean most of today FM music, (I'm sorry Britney )
Does it better the sound of the GC, or there is nothing like a passive pre?
No schematics? Not even a small one for all of us, green of envy of your sound? please...

 

[carlosfm]

Yes. I experimented a bit with the BUF outside the feedback but it (naturally) was noisy as hell. I've decoupled both the opamp & BUF's supply rails with electro's in parallel with ceramics at less than 2mm from the pins. Maybe my cheap junk closedback ultra-sensitive 18ohm headphones made it more obvious. The noise was much less on a set of 30year old Sennheiser openback insensitive headphones. I always use my junk headphones for testing headphone amps because it's very easy to spot noise with it.
Pierre

My Beyerdinamic headphones are very revealing and I hear absolutely no noise.
I test first with some cheap Sennheiser headphones and same result.

And this is without a box, laying on the floor, with non-definitive wiring...

 

[carlosfm]

Ok, the schematic

Ok, Raka, here it is.
Sorry for the confusion, in fact the schematic I based to do the preamp is this one, not the one on the datasheet (they are similar).
I put this here without the owner's permission, but as this was taken from a thread here on the forum...

Basically I don't use R12 (this circuit was for an "active cable"), R4 is 4.7k and I didn't use R11, just a direct connection between pins 1 and 3 of the BUF634 (wide BW).
PSU bypassing from each rail to ground on each chip is done with 100uf electrolythics, bypassed with 1.0uf ceramic under the circuit.
What do you need more, Raka?

Oh, and it betters vastly the pre-amp seccion of my Nad amp, that I was using before, and that is tweaked by me (OPA2132 op-amps, properly bypassed).
I didn't try a passive pre.
I use my GC as a power amp, and a passive pre is not for me.
I would use just a pot if I had a GC as an integrated amp.
Come and hear the dynamics, detail, precision...
It kicks.
I call it slam.
And did you see my DIY Cat.5 cables?
Kimber-style.

 

[carlosfm]

I forgot to say in the previous note that I didn't use the 1uf input cap.
No cap.
0.2~0.3mv DC offset.
The circuit is basically the same for the headphone preamp, only the gain changes.

 

[Raka]

Thanks,

It looks simple and very good. This buffer can be used with almost any amp, right?
I think I will try it, as soon as I get the IC's. So if you recommend this one to my GC, I'll have to save space in my current box, or maybe get the trafo out of it...

What do you use for volume control?

How did you make the speaker cable?
I use a TNT cat5 cable, and UBYTE1, and sound very good , but the best thing is that are dirty cheap

 

[carlosfm]

Thanks,

It looks simple and very good. This buffer can be used with almost any amp, right?
I think I will try it, as soon as I get the IC's. So if you recommend this one to my GC, I'll have to save space in my current box, or maybe get the trafo out of it...

What do you use for volume control?

How did you make the speaker cable?
I use a TNT cat5 cable, and UBYTE1, and sound very good , but the best thing is that are dirty cheap

Raka,
Make a separate preamp.
Or let your GC alone with the pot, it's good.
I did it this way because I think a pre is a pre and an amp is an amp and as I'm doing it, I like to keep it separate.
My pre may work well with any power amp.
It works very good with my GC.
And I would fiddle to put all those RCAs in the same box of the GC.
The volume control is an Alps Blue, I said it on the first note.


What speaker cable?
My speaker cable is Kimber 4TC...
I was referring to the interconnect cable that is soldered to the preamp board on the picture.
Anyway, I have Cat.5 as speaker cable on one of my systems and I think it's very good.
A cable that transmits very high frequencies over a 100-120m distance has to be good.
Did you know that a cable, to be Cat.5 (that's a norm), has to use high-purity OFC copper?
Yes, it's OFC copper.

 

[carlosfm]

I really think that together with your planned small GC, you will have a minnuscule great gear!! Which case are you using?

No, this pre goes with my one and only PC Gainclone.
The ultra-small GC I'm making with LM1875s is maby for my computer.
I plan to fit it on the smaller box I can find that can accept it, a selector for 3 sources and a volume pot.
It may be a system to take for the vacancies, with a portable CD player and very small speakers.
I think that any PSU that I can get (batteries or transformer) will be bigger than the amp.
On a 5x10cm board I have the two channels.
Dual-mono , with completely separate inputs for signal and PSU.
And with the 1000uf caps, and 4.7uf polyester input caps.
How about that?
I'm speciallizing myself in miniaturization.

 

[Nuuk]

Hi Carlos, thanks for the link. I don't know why I haven't spotted this thread before!

Nice neat work there! I'm not surprised that it sounds good as you have obviously taken a lot of care.

You said the opamps were 'hand picked'. Were you joking or do you mean that you have enough OPA627's to pick and choose?

 

[MBK]

Interesting. I've built a headphone amp using an OPA2132 with a BUF634 as explained in the BUF634 datasheet and, no matter how well regulated my supply, short & shielded the connections and decoupled the opamp, there always was excessive noise. I finally gave up and built a discrete headphone amp.

Hmmm, I built something similar as the headphone output of my preamp. Similar to here, no input caps, wide BW, OPA2134 and BUF134, Rout about 100R. Sounded definitely too "sharp" with Rout as 10R.

I can hear a lot of noise too on my Beyerdynamics, but ... it must all come from the sources: there is a lot of difference between noise of different CD's / tracks, you can easily hear the differences even between channels, you can hear when the mixer turned up the faders... and no noise between tracks. So it definitely works - it's just very revealing.

The things I am not so sure about is firstly, the fact that the size of the output resistor changed the sonics so much nags me as to the cause - parasitic oscillation? And if so, w/o a scope (sigh) how do I know if it's gone now or just reduced? ...

Incidentally: ever hooked up your GC to headphones? I did that with an OPA548 chip amp. Hooked it up in Mono to the beyerdynamics from the big 375VA unregulated supply (use volume control!!! of blow the headphones to kingdom dot com). Amazingly sweet to listen to! Sweeter than the OPA2134/BUF134 combination and yet with dynamics to rip your ears off. With no output R at all. Possibly less revealing, but very musical, without the slight "sharpness" of the OPA2134/BUF134.

 

[trespasser_guy]

I am listening to my OPA2132P+BUF634 (in WB) amp right now.

I get -.2mVDC on one channel and -2.2mV on the other. With my 300-ohm cans (HD580) this doesn't matter much. Soon (when I can gather enough money....) I will build an amp with OPA637 and BUF634T. Perhaps I will include a tube buffer pre-stage. I will use about 50,000uF in the PS, regulated, of course. With my current amp, I get no noise unless no source is connected. BUFs get a tad warm since I connect BW directly to V- with no resistor. I want to ensure WB at all time. lol

 

[carlosfm]

You said the opamps were 'hand picked'. Were you joking or do you mean that you have enough OPA627's to pick and choose?

Yes, I have enough.
I tested several until I've got minimum offset and equal between channels.

 

[carlosfm]

Hmmm, I built something similar as the headphone output of my preamp. Similar to here, no input caps, wide BW, OPA2134 and BUF134, Rout about 100R. Sounded definitely too "sharp" with Rout as 10R.

I have 120ohm 0.6w 1% film resistors in Rout, on the headamp.
The sound is very detailed and never sharp.
The headmap has a gain of 11.
As for the bass, it's a bone-crunching experience.
It really kicks.
But no flappy bass, it has an impressive definition.