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Old 7th September 2003, 02:56 PM   #11
EC8010 is offline EC8010  United Kingdom
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Thanks for that, Frank, it's much prettier. The output coupling capacitor is rather large, but I'm assuming a 10k load resistance and 1.6Hz cut-off frequency. 4u7 would probably be perfectly satisfactory.
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Old 7th September 2003, 05:40 PM   #12
PRR is offline PRR  United States
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I agree with nearly all the above.

The great thing about the TL072 is you used to be able to get all the parts at Radio Shack on a weekend, and for this type of application 99% of listeners on 99% of systems could not tell a TL072 from a perfect buffer. But RS stocks fewer chips in-store now, and fashion changes. If you have to order parts, folow the dictates of fashion and use something "not so old"; OPA2134 is well-regarded.

Given time and some cash, I won't discourage a tube. They are fun. I feel that it is a lot of work to get "just a buffer", but it is about as easy as tubes get and probably the easiest topology to de-bug.

Frank's 2-triode (per channel) White Cathode Follower is exceptionally good at low impedances and does tend to cancel even-order distortion. (However I suspect the plate resistor should be a hair smaller to optimize it, more like 450Ω. Not an issue for line level use at this supply voltage though.) It is a mind-bender to analyze and can be difficult to de-bug if it does not work first-try, and does mean two sockets,

EC8010's plain old Self Biased Cathode Follower is simple, and fault-free in this use. If you prefer, you could run it on a lower voltage, like 150V, without significant change. BTW: not only is the output cap a tad "generous", that input cap could be a lot smaller (the 1Meg grid resistor is bootstrapped by the cathode; the input cap "sees" about 20Megs). The Self-Bias Cathode Follower has better supply rejection than the White Cathode Follower, so a minimalist rendition of the SB CF can be a lot cheaper than a WCF.

> the source is a cdp, max output impedance is 10k. The problem is more about (possibly long) cable capacitance issues.

10K? Gak. At 10KΩ, 30 feet (10M) of ordinary cable will roll-off at 16KHz. Even a 6ft/2M cable only goes to 75KHz at -3dB, -1dB around 37KHz. Magic super-cables might go 50% further for the same response; some over-shielded cables might do worse. You sure do not want to take that source across the room without a buffer sitting in the same rack.
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Old 7th September 2003, 06:47 PM   #13
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thank you so much everyone, I've now enough stuff to think about carefully.

fdegrove : your design is really appealing. I read the original topic where it is discussed in details. In this post, you stated that the PS was critical to the quality of the sound. Budget and skill could make difficult for me to use yours right away. Would a more basic one do the trick till I can afford a better one ?

PRR : sadly, there's nothing as practical as Radioshack in Belgium, I'd have to order a bit. To be honnest, I'm now have to choose between a tube buffer which looks a bit more fun and a more classical BUF-04 buffered passive preamp (stereophile like), with a welborne labs PS-1. Since I'm building, I'd like to build something i don't wanna upgrade tomorrow morning.

EC8010 : what do you mean by "HT" in "A 300V HT supply will work for most circuits" ?
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Old 7th September 2003, 09:45 PM   #14
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Hi,

Quote:
Would a more basic one do the trick till I can afford a better one ?
You could do that although the series regulator only adds 2 noval sockets, one pico7 socket a few resistors and caps, plus a EL86, ECC83 and a 85A2/5651 glower...

This would amount to roughly 25 Euro if you'd had to buy the lot, worst case.

Anyway, I'd keep 0-250V secundary + some real estate for the 6.3V xformer for future upgrades in case you'd opt for a pi-filter now until the budget allows.

FWIW, when using premium components, this circuit comes as close to no sound of its own as I can get but it ain't exactly "low budget".

While the PSU is always important, the PSRR isn't any worse than most stacked triode configurations a la SRPP.
All too many times I've seen the SRPP bashed because of a badly implemented PS but that's another chapter.

Quote:
EC8010 : what do you mean by "HT" in "A 300V HT supply will work for most circuits" ?
If EC8010 allows me, HT is Eurospeak for what our U.S. friends call B+.
IOW it's the high tension ( haute tension en francais) rail.

There's a raft of other confusing terms in tube/valve land you'll discover soon enough.

BTW, I was in Verviers a couple of times this summer, lovely place.
I somehow doubt however, given your excellent English, that you're a native?

Cheers,
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Old 8th September 2003, 08:02 AM   #15
00940 is offline 00940  Belgium
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Quote:
Originally posted by fdegrove
BTW, I was in Verviers a couple of times this summer, lovely place. I somehow doubt however, given your excellent English, that you're a native?
I'm a native, I just spent last year studying in the States (CU at Boulder, CO).

Going back to the topic which brought me here, I'm gonna make a more careful evaluation of your design's costs. It's true there is no choke in there and iron is typically more expensive than glass. Diyparadisio sells a transformer with a 275V@130mA and 2X 6.3V@3.6A for 41.5. Is it suitable ?
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Old 8th September 2003, 12:30 PM   #16
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Hi,

Quote:
Diyparadisio sells a transformer with a 275V@130mA and 2X 6.3V@3.6A for 41.5. Is it suitable ?
That's a tad high but it can be made to work.

As you may have noticed I used separate xformers for each channel, 0-250V+ 6.3V...
I don't quite remember the VAC rating but that's not too hard to find out.
The 6.3V winding was only used to feed the regulator's heaters.

A separate 15V xformer/channel was used to feed the 12BH7A with regulated (7812 or LM317, for instance) 12.6VDC.

If we stick with Bennys' xformer only and don't use the regulators for the 12BH7As you could use Pi filtering to burn off the excess voltage.

The 2*6.3VAC windings could be put in series and rectified (not enough DC V available to regulate), again pi-filtered to adjust for 12.6VDC for the heaters.

Alternatively you could use a voltage doubler ( there's way too much current available anyway), filter that and regulate it for 12.6VDC out.

Cheers,
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Old 8th September 2003, 12:55 PM   #17
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by fdegrove
The 2*6.3VAC windings could be put in series and rectified (not enough DC V available to regulate), again pi-filtered to adjust for 12.6VDC for the heaters.
12.6V rectified should produce about 16.5V, which is enough to regulate to 12.6V if a low drop-out regulator is used. However, rather than use the windings on the HT transformer, I would use a separate transformer to avoid HT switching spikes appearing on the heaters.
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Old 8th September 2003, 02:38 PM   #18
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actually, there's two interesting xformer at diyparadiso, at the same price :

150V/80mA 225V/80mA
and 2x 6.3V/2.5A 30V0.25A
or
2x75V 130mA & 275V 130mA
and 2 x 16V 0.6A & 2x6.3V 3.6A

(see http://www.diyparadiso.com/price/stock-transfo.htm )

I've printed your posts about "the ultimate preamp" and I'm reading them carefully.

By channel, we need at the outputs of the transformers :
A. 1x 250VAC (at about 300mA), giving the 305VDC of the B+
B. 1X 6.3VAC (at 1.2A), giving the 6.3VAC heating the regulator tubes (EL86 and ECC83)
C. 1X 15VAC (at about 300mA), giving the 12.6VDC heating the signal tube.
Right ?

So I need 6 transformer or could I use a same xformer for both channel, if the regulation was seperate ? How powerful should be those transformers ?

I'm a bit affraid all this iron will bring me out of mu budget. I should perhaps stick with ec8010 design and use a less ambitious PS. Is there anything you would advice as a good basic PS ?
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Old 8th September 2003, 08:19 PM   #19
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Hi,

Quote:
So I need 6 transformer or could I use a same xformer for both channel, if the regulation was seperate ?
No, not really.
If we step back to the xformer Benny is offering:

R-core tube amp pre amp secondary :

2x75V 130mA & 275V 130mA
2 x 16V 0.6A & 2x6.3V 3.6A

While it sports 2*75V that are useless in this case, it still has what is needed.
The only disadvantage is that the diode rectifiers can spoil things as EC8010 has pointed out.

Assuming you don't have a scope handy these nasty spikes can be tough to get rid of.

So, if you want to avoid that, the 2*16V (15's fine too) can be on the same xformer.
The 2*6.3V windings for the regulator valves, they're fed with AC, and a single 0-250V winding would suffice for both channels.

In my preamp I used xformers from the RS components catalogue, I'm not sure they still carry those 0-250V ones though.

To summarise: it can be done with just two xformers if you can find the right ones.

Hope this helps,
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