META-variant with BUF634 questions
Oct 8, 2002 at 2:47 PM Thread Starter Post #1 of 15


Supafly & The Funky Pimps
Sep 13, 2002
Hi there- I've been planning a rig with two single-channel OPA627s, buffered with BB BUF643s. At this point, I just did a quick four-socket layout on RS 3-pad protoboard, and followed the META42 schematic from Tangent's site as a starting point. I followed it very closely so far, the exceptions being not adding a current source (yet... it will come though). I also made R2 500k, as I plan on using a 50k pot. The coupling caps are .22uf polypropylene. I'm powering it through a seperate bench-top linear power supply at this point. (Agilent)

The good news is- it works. There is some hum, which is likely due to the funky temporary leads I have running to input jacks and soforth. It should go away just fine when I use shorter shielded cable that isn't running alongside power leads. The buffers and amps get only slightly warm to the touch, maybe 105f at the most. I did try some single-channel OPA132s before testing the spendy 627s, and they got HOT immediately. I don't believe BB recommends them to be buffered with the 634s in wid BW mode.

However, the gain seems to be outrageously high. There is no volume pot at this time, but I am testing on the headphone jack of my portable and using it's pot. I can't go above 1 on the pot- it's about as loud as my other amps at almost full volume! This may also be contributing to the slight hum.

Again the values throughout are the same as in the linked schematic. I understand that the BUFs are not the same beast as the EL2001s. I do have them in wide bandwidth mode, and am aware of the different pinout. I don't think I can use Tangent's gain calculator for this, as the present gain is WAY different then a META with EL2001s.

Any thoughts on appropriate R values here?

Also, PPL mentioned in another thread that I could use the jfet cascode current source for this amp, but if AC powered, I should use diff. transistors to achieve approx. 5ma current drain. While I understand why, I don't know how to go about choosing a proper model jfet. There are many similar models in that Fairchild family.

Thanks in advance. I have heard good things about the sound of the OPA627s paired with the BUF634s, for those who like the BB sound.
Oct 9, 2002 at 2:42 AM Post #2 of 15
Attached is The Schematic of My Latest design using the BUF-634. The OPA-627 is quite stable on my version Evean with the Buffer in the low bandwidth mode with the widebandwidth resulting in Better sound But Reduced battery life Like 50 Ma an Stack of Three BUF-634s and OPA-627 and at +/- 10 volts the Opamps are not evean warm. So i would use the attched circuit Including all the Powersupply Parts. I stress enougnt on these fourms the Requirement of a Quality low noise power supply on any High resolution Audio Component More accurate Signal circuits demand the finest in supply Quality. as for Fets select somthing with an Idss specification around 5 Ma like the MPF-102 Might work. Just measure the actual current when Installed by measuring the Voltage drop across the series resistor
Oct 9, 2002 at 7:31 AM Post #4 of 15
That's Right KurtW it is in same fam as the 5484 i use. but as kurt has pointed out higher Idss this should work fine.
Oct 9, 2002 at 12:03 PM Post #5 of 15
Thanks for the information, gus- I really appreciate it. I ordered a handful of 2N5458 and 2N5485 yesterday, oddly enough. Sounds like the 2N5485 may be the best bet. I got the 2N5458 on a hunch- it may be totally unsuitable??? I hope not, as I have 30 of them now!

Thanks for the schematic, PPL. Just what I need to get me off the ground here. Just one question: Where is the output? Take it right off the BUF634?

I do tend to overbuild a power supply, and they are always silent. At least the ones I use are always silent... I did try re-regulating a laptop powersuply a while ago, it was horribly noisy still.

Thanks for the guidance.
Oct 9, 2002 at 12:07 PM Post #6 of 15
This is good you start off with a now noise supply but the added circuits i use are to isolate the gain stage from the output stage and thus prevent contamination of the gain stage rails from the back emf of the Headphone driver.
Oct 9, 2002 at 12:09 PM Post #7 of 15
Ahhh... that makes sense. Thanks for the added info.

One other question: R13 in your schematic- it says 4.7. Should that be 4.7K, or is it really a 4.7ohm?

I also understand that the output would just come off the BUF, just not drawn on. Thanks, PPL.

Oct 9, 2002 at 8:12 PM Post #8 of 15
PPL- not sure if you got my PM response. You can send mail via my profile as well.

Thanks very much!
Oct 10, 2002 at 11:52 AM Post #9 of 15
It's working fine, now. Seems it was still damp from washing flux away when I first tested it (oops). It's good now. I'm still going to fiddle with the outer gain loop, and add a current source when I get my jfets from Mouser.

It sounds nice, warm and smoth, with quite a bit more punch than a single stage amp. Hard to compare it to the META42 (virtual clone), but it's just a bit more tube-like where the AD and Elantec META42 is more articulate in the detail. Not that the BB buffered variant is mushy, it's just not as 'crispy', if that makes sense to anyone.

I'm running a pair of OPA627As right now, but plan on plugging in some OPA637As soon. I've read some positive remarks regarding them in the archive, from people who don't particularly even care for the 627s. I'm curious.

I'll try to get some pics of this board up this weekend.
Oct 11, 2002 at 12:17 AM Post #10 of 15
If the Buffer is operated wideband mode the OPA-637 will be stable. May require finetunning because the wrong implimentation will give quite brash sounds. Regarding the differences between the AD's and BB's i came to the same conclusion the Midrange is quite distent with the BB's but this can be a blessing on some recordings. The BUF-634 really likes to be Parralelled to sound it's best I like Three.
Oct 11, 2002 at 3:36 AM Post #11 of 15

Yes, I think I need a bit of tweaking to get the 637s to sound right... they are a bit erratic, and my BUFs are in wide mode.

That said, I did stack them (BUFs) two high tonight, and the sound is improved. The 627s are in the amp right now. I also added the current source (cascode pair of 2N5484s, with 1k resistors). That has made the most difference thus far. I may swap them out for something with more drain, but am still waiting for parts. I also need a beefier power supply, as you noted I would! This thing sucks a lot of juice- the battery power supply I'm running it on has a BUF634 as well, and I had to stack that one also. The buffers are warm, but not hot, and the amps are not warm.

Tomorrow, I'll build another pair of buffers stacks, with three each, and swap them in and out a few times. The EL2002s in my META ar two high right now, I understand they might have to go four high to get any more out of the setup... perhaps not worth it.

It's turning out to be a decent amp. Different qualities than the META... I like to listen to jazz on it. Stan Getz and Dizzy Gillespie at the moment. The BB amp and buffer combo does definitely lend a tube-like warmth. Don't get me wrong, I also like very detailed sound, articulated- and the META42 does deliver in spades on that note. This is just different, and I'm liking it more and more.

Thanks for your help on this project!
Thanks also to Tangent for the matched jfets for the cascode. I have a bag on the way so I can start sorting some for myself. I hate to keep asking you to do that!
Oct 11, 2002 at 6:47 AM Post #12 of 15
The Chips from any given manufacture tend to have a Unique sonic signature throught all devices of the same brand. thus a BB Buffer will have a sonic signature similar to BB opamps. As far as the sound of the OPA-627/BUF-634 it as you say warm and tubelike. subbing the AD-744 for the OPA-627 and using the comp pin on the AD-744 the sound gets down right lush and i know some hear at head-fi that loves this Combo. I emailed you a schematic on that version hope you got it.
Oct 11, 2002 at 12:52 PM Post #13 of 15
LUSH? That sounds like a so-called Good Thing to me!

Thanks, ppl, I did get your emails, and I appreciate it. I have to order some 744s today, it seems.

Have to run for now, but I'll be sure to keep you posted.

Oct 12, 2002 at 7:34 AM Post #14 of 15
To answer your first question R13 in My posted schematic is a ground loop breaker, and is 4.7 Ohms.

Yes Lush as in evean more of the tube like qualities you mentioned and I hear some Call The sound of Sing End Class A Triode 300 B in paticular as sounding Lush. But I also hear tube gear called Euphoric and that to me implies coloration unless that is the Fealing you get listening to sed unit. Hope this makes more sence now.
Oct 13, 2002 at 3:06 PM Post #15 of 15
Thanks for the clarification of the R value, ppl. I thought it was correct, but it's an unusual value, for sure.

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