Surprised by solid improvement in my SE DHT amp

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RhythMick
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Surprised by solid improvement in my SE DHT amp

Post by RhythMick » Thu Nov 14, 2019 2:28 pm

When I got into amp building a few years ago I did a lot of reading before I got around to building anything. I took away from that reading an aversion to the coupling cap which has until now strongly influenced my amps.

Last weekend I changed my DHT SE amp to cap couple V2 (71A) to V3 (300B). The interstage transformers were removed (Tribute Finemet), I subbed a LL1667 choke as anode load for V2, grid leak resistor for V3 and a cap between the two. The grid leak were 88K 2W Allen Bradley while the caps were inexpensive 1uF (Cornell).

Against all expectations the sound is significantly better. More solid and musical throughout the range, much better sound stage ... The word Geoff and I kept coming back to was "immense".

Just goes to show how important it is to challenge your own assumptions, which for me at least is a large part of the pleasure of DIY. I think the improvement will be coming from the choke, which gives 200H compared to the 60H of the Tribute primary. Looking forward to doing the same with V1 (26) to V2.

I can barely wait to bring this to Owston, I need some independent feedback. Anyone near South Yorkshire fancy a listen? I'll provide food and beer.

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Mike H
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Re: Surprised by solid improvement in my SE DHT amp

Post by Mike H » Thu Nov 14, 2019 3:35 pm

Yep. My faves for this sort of job are audio grade polypropylene and polystyrene. Pretty much anything else I can hear and don't like. My fave audio polyprop is ClarityCap, made by ICW (International Capacitors Wrexham) and sold by Hi-Fi Collective. Also I've used an alternative made by LCR (in South Wales I believe), that Maplin used to stock. Yellow wrap with blue resin in the ends and generally smaller phyisically than equiv value ClarityCap. However the LCR's often seems to be lacking in bass for some strange reason. Otherwise OK.

EDIT (appendix) I want to have another go at my 801A SET but this time I want to do what you've done, anode choke on the driver and a ClarityCap for coupling.
 
I have boxes and boxes full of components, but never the right ones!

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IslandPink
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Re: Surprised by solid improvement in my SE DHT amp

Post by IslandPink » Thu Nov 14, 2019 3:39 pm

I've never been convinced about interstages in SE amps. The limited inductance of the gapped transformers seems like a major problem.
Also unless the tranformer is super-good, they always lose a bit of low signal level stuff, compared to a good ( film/foil ) cap.
I had a three-stage amp, and it was ECC40 direct-coupled to a 45, which was choke-loaded and cap-coupled to a 300B XLS. ( Very nice - but i still went back to a 1-stage pentode driver in the end )
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

RhythMick
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Re: Surprised by solid improvement in my SE DHT amp

Post by RhythMick » Thu Nov 14, 2019 4:12 pm

Interesting. I've started with the view that I'll use relatively inexpensive coupling caps to experiment and get the value right, but I'm skeptical about the effect on the sound of different makes. I'm prepared to give different makes a listen once I've got the value right, but I'm expecting the £7 Audyn MKP, the £4 Cornell-Dubillier, the £50 silver/gold in oil to all basically sound the same. I'll prepare to be astounded.

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Re: Surprised by solid improvement in my SE DHT amp

Post by Wolfgang » Thu Nov 14, 2019 4:25 pm

And with typical OTLs it is even more critical as they have to use a lot of NFB. Whatever is changed by the input/driver stage (caps, chokes etc), diverting from the original input signal, is "amplified" by the NFB on the opposite side of the frequency range where it happens.
After I had reduced dramatically the NFB of the 300B SE OTLs moving from 8 to 15ohm drivers the rather typical sound of 300Bs in the lower midrange started to reappear a bit in its typical warm, rich tone colors for which we know the 300B. That was a key experience for me to decide my future course regarding OTLs.

Other people had similar experiences.

http://glowinthedarkaudio.com/transcend ... t-otl.html

RhythMick
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Re: Surprised by solid improvement in my SE DHT amp

Post by RhythMick » Thu Nov 14, 2019 4:28 pm

IslandPink wrote:
Thu Nov 14, 2019 3:39 pm
...it was ECC40 direct-coupled to a 45, which was choke-loaded and cap-coupled to a 300B
I plan to try 45 in that middle position very soon

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