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Old 27th October 2009, 07:48 PM   #71
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Quote:
Originally Posted by andyjevans View Post
Kevin refers to red LEDs, and I just tried those in place of a cathode resistor. Initially I was excited - very clear and dynamic. But there was an edge to the sound I couldn't live with, and I took them out and put in a wirewound cathode resistor. I found that fine - the smoothness returned and sound was only marginally less clear and dynamic.
This is very like the reaction I felt to using red-LED biased current source in the shunt-cascode driver for my 300B-SE. I took it out again and used cross-coupled transistor current source.

Curious
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Old 27th October 2009, 11:03 PM   #72
kevinkr is offline kevinkr  United States
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Originally Posted by Rod Coleman View Post
This is very like the reaction I felt to using red-LED biased current source in the shunt-cascode driver for my 300B-SE. I took it out again and used cross-coupled transistor current source.

Curious
Ring of two type CCS? This is my general preference as well for bipolar transistors. In general I have avoided CCS in the audio path of my own equipment. I am experimenting a little with some of the IXYs devices - SY, Doug and a couple of others think very highly of these which has my curiousity piqued.

I was thinking perhaps I had a specific preference for bias approach but I think I might be almost close to agnostic. I use cathode bias with tubes like the 6J5/6SN7/6SL7 almost exclusively, fixed (grid) bias in power stages and with DHT types in general or and some high transconductance types like the 5842. I have also employed LED bias with 5842 and D3A with choke loads or tube based gyrators. I've found the LED bias sounds good in these applications sans CCS. Using high transconductance types with LED bias in particular means that you need to pay close attention to matching and target plate current vs supply voltage (since I don't currently use CCS).

Fixed bias seems to work really well with low transconductance dhts and results in reasonably predictable Ip which generally is not the case with higher transconductance types which need to be tweaked to achieve the desired operating point.

I guess there are about as many ways to do this as there are designers.
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Old 27th October 2009, 11:38 PM   #73
kevinkr is offline kevinkr  United States
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Originally Posted by andyjevans View Post
Hi Kevin,

Seems like you have some nice amplification there! What speakers are you using, and how did you make that choice? Interested to know!

andy
Hi Andy,
Like most of my system I designed and constructed the speakers myself - it was, so to speak "the final frontier" They are Onkens which are based on the old Jensen Ultraflex design. Mine use Iconic 165-8G 16" HE woofers bought new (basically the same as an Altec 515-8G) and vintage pro JBL 4333 mid horns with acoustic lenses/diffusers on them and JBL 2402 annular ring radiators. These are all tied together with an external 2nd order Butterworth x-o using generic Madisound air core inductors and Clarity SA film caps. You can find more details on several of the older Onken threads on our speaker forum and some pictures in my profile. They are efficient - the bass bins just a tad under 100dBspl@ 1W/1M and the mid/high horns are over 110dBspl @ 1W/1M allowing the use of a much smaller amplifier if I decide to bi-amp - who knows bass by GM70, highs by 300B? It's just too much fun!

Edit: I didn't mention that the speakers were designed for use with SE amplifiers with a relatively high output impedance in the range of 2 - 3 ohms. This includes the cabinet design & bass alignment and the X-O electrical design as well. It works quite well I am happy to report, but it is the second X-O - the first one was marred by a few errors in the design analysis.

FR wise in room they are better than +/- 2dB from 100Hz - 10kHz, and are down 3dB around 35Hz, and 16kHz. (the annular ring radiator is great to 15kHz and starts to roll off at that point) I used Arta and/or audiotester to measure in room response with a once calibrated behringer mic.
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Last edited by kevinkr; 27th October 2009 at 11:47 PM.
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Old 28th October 2009, 09:36 AM   #74
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Hi Kevin,

Thanks for that! My own room is too small for such speakers. I have Apogee Caliper sigs. Love them, but may downsize to a Jordan JX92 setup, maybe TQWP, if I can make it sound good enough. The Apogees need a PP amp which is frustrating.

andy
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Old 28th October 2009, 03:37 PM   #75
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Hi Andy, have you tried a para-feed solution in W.E. style? The only difference would be in the CCS, solid state in your case vs the original inductor.
You could use a relatively cheap and very good Sowter:
Sowter Type 3232
To complete the design you could go for the Sowter transformer attenuator at the input.

Cheers,
45
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Old 28th October 2009, 04:34 PM   #76
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Hi 45,

That's an option - thanks for that! I could try parafeed with a Lundahl OPT just to see. I could try choke loading or active load - did you find a preference if you tried both? Can you remind me of the WE setup with a SE stage? I'm thinking of the PP version of a cap between shared cathode and HT of transformer.

andy
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Old 28th October 2009, 05:47 PM   #77
45 is offline 45  Italy
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Quote:
Originally Posted by andyjevans View Post
Hi 45,

That's an option - thanks for that! I could try parafeed with a Lundahl OPT just to see. I could try choke loading or active load - did you find a preference if you tried both? Can you remind me of the WE setup with a SE stage? I'm thinking of the PP version of a cap between shared cathode and HT of transformer.

andy
It is the driver stage. More precisely the para-feed driver of the WE 92a, for example:
Western Electric - Rosetta Stone for Triodes
You just put a CCs (or a choke) in place of the 100K resistor and play a little bit with the caps. However I think you need the input trans to get the best out of it.

This is a recent one (look at the input stage): http://www.nutshellhifi.com/GarysAurora.gif

Another interesting schematic is that of the Klimo Beltaine. The output stage uses a SE 300B through 1/2 primary of a PP output trans, the DC induction is balanced using an EL 34 connnected on the other half. You could use the same principle for the preamp.
I don't have schematic right now but I have seen it somewhere. This little 5W amp was really delicious. It is a pity it has been discontinued.....

Cheers,
45

Last edited by 45; 28th October 2009 at 05:50 PM.
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Old 28th October 2009, 06:03 PM   #78
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Default Hot!

There is one way of biasing a DHT not mentioned here. When using the heatercurrent to get the necessary cathode biasvoltage no decoupling cap will be needed.

This is just an HOT idea, have not tried it myself yet. The cathode-resistor will dissipate over 8W! Think I have seen it somewhere before and Im going to use it on a small 3A5 parafeed.
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Old 28th October 2009, 07:05 PM   #79
45 is offline 45  Italy
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P.S.
Andy I have not tried the para-feed in a preamplifier, yet. I always prefer an integrated amp to a pre+power amp.
I have experience with LC coupled driver for a 300B SE. It was very nice indeed.

Cheers,
45

Last edited by 45; 28th October 2009 at 07:14 PM.
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Old 28th October 2009, 07:06 PM   #80
45 is offline 45  Italy
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ops...I sent it twice!!

Last edited by 45; 28th October 2009 at 07:14 PM.
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