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Post by snewt on Dec 4, 2017 18:30:30 GMT 12
As some of you may know I've been without a preamp for nearly two years now. I bought a volume control recently and this has proved to be surprisingly good but I can't help but wonder if a buffer stage may work better.
Have any of you had any experience of the djw b1 buffer? It would give me the ability to switch between 2 inputs, give me one volume control, and improve the impedance mismatch. All for around $200.
It would go between my phono stage and CD, and my Williamsons.
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Post by Owen Y on Dec 4, 2017 18:53:18 GMT 12
Remind us.... What is your source & what vol control are you using? Where on the dial do you have your usual volume setting ?
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Buffer stage Dec 4, 2017 19:49:59 GMT 12
via mobile - Edited Dec 5, 2017 5:30:17 GMT 12 by snewt
Post by snewt on Dec 4, 2017 19:49:59 GMT 12
I've got the Aesthetix Rhea phono stage, which has selectable gain from 38db to 75db in 6db steps, and a Meridian 200/203 CD player.
The volume control is a dual mono affair with a couple of volume pots and I don't know what else.
I tried connecting the phono stage directly to the power amps but got hum as the amps powered up. Gain isn't a problem it would seem, so I'm wondering if I can avoid buying a megabucks preamp.
I'd appreciate the thoughts of the forum regarding using a buffer stage in place of a preamp.
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Pundit
Post by Gryffles on Dec 4, 2017 21:43:34 GMT 12
Don't mean to be a wet blanket but I reckon an active preamp is nearly always better IMO.
I missed my DHT pre (the one we listened to) so much that when I found another I purchased it from the US.
Your welcome to have a listen in your system to see if it does make a positive difference.
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Post by Owen Y on Dec 5, 2017 6:57:27 GMT 12
Ah OK. I use an active linestage myself, but passive can work satisfyingly for some folks. Extra dynamic headroom usually vs additional circuitry in signal path. Do you know if the volume pots are wiper type pots, or stepped attenuators? Also what impedance pots they are? eg. 15k 50k 100k.
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Post by snewt on Dec 5, 2017 16:30:37 GMT 12
Thanks Ben. That does seem to be the prevailing wisdom.I guess the best way is to suck it and see.
Thanks for your generous offer. We'll have to do that soon.
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Post by snewt on Dec 5, 2017 16:38:49 GMT 12
Hi Owen, pretty sure that they are wiper type pots but I have no idea what impedance they are sorry. Not much help I know.
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Post by Owen Y on Dec 5, 2017 17:48:57 GMT 12
That's OK - where on the vol control do you usually wind it up to, for normal Phono/CD listening?
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Post by Owen Y on Dec 5, 2017 18:48:24 GMT 12
Hi Simon... - Perhaps I'll move this thread to 'Amplification', if OK with you? - I don't recall, did you inherit Ken's 211 SE amp? (Your avatar image?!)
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Pundit
Post by Gryffles on Dec 5, 2017 19:43:47 GMT 12
Thanks Ben. That does seem to be the prevailing wisdom.I guess the best way is to suck it and see. Thanks for your generous offer. We'll have to do that soon. No worries. My pre is heading down Clear Image Audio to get converted to NZ voltage tomorrow so will be back in a couple weeks I hope.
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Post by snewt on Dec 5, 2017 20:56:17 GMT 12
That's OK - where on the vol control do you usually wind it up to, for normal Phono/CD listening? For the phono stage set at 62db probably 8 o'clock with 7 being zero, and 9.30 for CD. As I say, no problem with gain.
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Post by snewt on Dec 5, 2017 21:00:54 GMT 12
Hi Simon... - Perhaps I'll move this thread to 'Amplification', if OK with you? - I don't recall, did you inherit Ken's 211 SE amp? (Your avatar image?!) Yes I did get the 211 amp but it has developed a fault - after switching on the idle current is too low - so I've put it away for the moment, and I'm using the Williamsons.
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Post by Owen Y on Dec 6, 2017 7:26:35 GMT 12
Ohh... but that's probably better than too high current  Did it sound OK in that condition, did both channel's tubes light up & play music?
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Post by Owen Y on Dec 6, 2017 9:48:54 GMT 12
Hi Simon - getting back to your orig query about a buffer stage, or whether one is necessary in your system..... The Williamson amp had an orig Input Sensitivity spec of 1.4v input for full power out (16wpc according to the 1947 articles). The Rhea phono stage will get your Shelter 501 or Benz Wood SL up to 1.4v with >70dB of Gain. However: (i) Your lspkrs probably need only 1 watt for loud listening, so a lower phono gain setting (say 56-62dB) would be adequate & might offer a bit better dynamics. (ii) The reason I ask about the volume pot resistance is that, sitting between the source & amp, it determines a load impedance for the phonostage & also the source imped for the amplifier..... If the Williamson is built to orig specs, it will have old-school high input imped/resistance (1M ohms or such) - which should allow you to run the vol pot quite high without imped mis-match to the amp input. (The output imped of the vol pot varies with vol setting, hence my Q about what is your usual vol setting, ie high or low?) Additionally, the specif of the vol pot can be critical with 'passive' set-ups like this, especially with a tube phonostage source - with its high output impedance of around 1k ohms. (Hence my Q about the impedance of those pots - I'd take the lid off if poss & take a peek.) Slightly complicated answer, sorry, to a not so easy Q 
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Post by snewt on Dec 6, 2017 18:25:13 GMT 12
Ohh... but that's probably better than too high current Did it sound OK in that condition, did both channel's tubes light up & play music? Unfortunately no sound either. I'll have someone look at it sometime.
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Post by snewt on Dec 6, 2017 18:46:08 GMT 12
Thanks for taking the time Owen. I've opened it up and the pots have 10k on them
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Post by colinf on Dec 7, 2017 6:22:04 GMT 12
Hi Snewt, Just read a review of the Aesthetix Rhea. Apparently it works best into a load 30k and above. That's because the coupling capacitor on the output is of limited size and 10k would start to roll off the deep bass a bit early. A passive volume control with 25k and above but not more than 50k would be better in this case. Any active buffer you might consider would be best to have an input impedance between 25k and 100k. Re your 211 amp; sounds like the high tension (HT) supply has failed, or a fuse inside blown if it has one.
AMR-iFi R&D
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Post by snewt on Dec 7, 2017 20:56:46 GMT 12
Thanks Colin, that's good to know. I might have to delve into the diy world and change the volume pots.
(And check the fuse!)
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Post by Owen Y on Dec 8, 2017 10:11:24 GMT 12
Further on colin's thoughts... From this pic, it looks like (my guess) the Rhea has one 2.0uF Rel-Cap on ea of its 4 outputs (2xRCA + 2xXLR) per ch. Connected to the 10k downstream vol pot load, LF will roll off -3dB at 8Hz - which doesn't seem too bad, but note that this means that the LF be 0dB flat to ~55Hz only. With a 30k load as recommended, the -3dB point will be 2.7hz & flat to ~19Hz (which is more what we'd like). (All according to the online RC filter calculator ) 
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Post by snewt on Dec 9, 2017 7:58:15 GMT 12
Reading from the same review, Colin, the reference to preferring a load of 30k is from the balanced output. Am I right in thinking that the rca outputs (which I use) require a different load?
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Post by Owen Y on Dec 9, 2017 10:39:16 GMT 12
Connected to the 10k downstream vol pot load,..... : The power amp input load R will be in parallel with the 10k vol pot, which in the case of say a modern 50k amp input, would result in ~8k loading => 10Hz -3dB rolloff & flat response rising to only around 70Hz. However with the Williamson's high input imped, it would barely contribute to any such HF rolloff. (An advantage of these vintage ccts - also less sensitive to IC cable characteristics  ) So it would also be useful to know what input R your Williamson was built with.
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Post by colinf on Dec 9, 2017 10:40:41 GMT 12
Re balanced vs single ended...Good point! The balanced outputs were measured as having an output impedance of 110 ohms at 1kHz and 3.8k at 20Hz. So a 10k load across the balanced output would roll off the deep bass a bit. The unbalanced output that you're using measured an impedance of 2k at 1kHz and 3k at 20Hz. So it looks like you're not too badly off frequency response-wise with a 10k load on the balanced output. Although it looks like a similar value capacitor was used on the single ended output as the balanced one. Just that the single ended output has a generally higher output impedance. What I thought was more interesting though was the distortion on the single ended output vs that on balanced. At the 8k load that John Atkinson used to measure the single ended output (why that load impedance I wonder?) the distortion was up around 0.186%. On the balanced output with a load of 100k the distortion was only 0.014%. Dropping the load on balanced output to 600 ohms increased it to only 0.05%. (But the bass would be severely rolled off at 600 ohms. He measured it at 1kHz.) Thus the distortion on the balanced output is inherently lower, from 3 times to up to an order of magnitude lower than that on the unbalanced output, depending on the load impedance. So in thinking of an even better buffer, I'd consider a highish input impedance (50k or above) balanced-to-single-ended converter. You could accomplish that with either tubes or solid state. A typical balanced-to-SE transformer would have too low an input impedance (it'd be less than 10k) to allow the bass to extend deep enough, and there might be other problems in using one such as the increase in distortion. That might offset the decrease in distortion from the balanced output vs the SE output to an extent, so there'd be a diminished advantage in doing that.
AMR-iFi R&D
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Post by snewt on Dec 9, 2017 11:27:45 GMT 12
Thanks again guys for your comprehensive answers. Here's a photo of the innards. Is it the little resistor across the input you are looking for? Can you make it out from that Owen? 
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Post by colinf on Dec 9, 2017 12:48:29 GMT 12
The input impedance is 470k in the picture.
AMR-iFi R&D
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Post by Owen Y on Dec 9, 2017 19:05:46 GMT 12
Yup I agree with colin, looks like 470k - which will not really contribute to the HF rolloff brought about by the 10k pot. What sort of pots are in the vol control? You could try some 25k to 50k ones, as colin earlier suggested. (Where on the 'clock dial' is you normal vol level? eg. 9 o'clock, 12 o'clock?, etc.)
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Post by Owen Y on Dec 21, 2017 19:35:07 GMT 12
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