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Old 8th July 2008, 02:42 AM   #1
Zap is offline Zap  United States
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Default Next Project - Darling Amplifier or 45

I've been having a lot of fun listening to my Simple SE lately and mixing up the tubes since I recently bought or found NOS gems to play with since I know it is working properly. I did have a few questions for anyone that has built a Darling, and some are also general. I'd like to build a smaller amplifier to use in another room, and one of the original designs that intrigued me was the Darling.

1. Has anyone done Darling monoblocks? Since these aren't a very large design, you could build two fairly small monoblocks. The obvious downside is two chassis and power transformers. Any thoughts are welcome.

2. I see a lot of these built using the black Hammond chassis. I love the look, but has anyone had issues with generated currents due to the magnetic properties of the steel?

3. The original design has shared parts between the two cathodes. Obviously, this couples the two stages together in some way. Good or bad reasons for separating them? Obviously the values need to be altered.

4. Output transformers - given the 750mW per channel output, I don't think frequency response should be an issue. Will more expensive transformers still sound a lot better? Has anyone built a fancier Darling? What transformers has everyone used? I've got the big CXSE Edcor's on my Simple SE and they sound really good. I'm wondering if better iron is available more reasonably since the power is so low. If I am not mistaken I don't want to go too big for the power output, or I will lose high end response.

I'm trying to keep from overspending on this amp, since I eventually would like to build a 45 amp and/or monoblocks. I've got a good priced source for some decent 45 tubes, and I may buy a set of Hawthorne Audio Silver Iris speakers, since I've heard great things about them. A 1626 or a 45 should be able to drive them to decent volume levels.

Any other thoughts, pictures, circuit modifications are most welcome. I've got access to a decent amount of tools at the place I'm interning this summer, so the chassis punching shouldn't be too bad.
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Old 8th July 2008, 03:15 AM   #2
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Quote:
I may buy a set of Hawthorne Audio Silver Iris speakers, since I've heard great things about them. A 1626 or a 45 should be able to drive them to decent volume levels.
I have a pair of Silver Iris, and a TubelabSE with 45's. The combination sounds fantantastic and the volume level is more than adequate for most music. This is the best sounding speaker - amp combination that I have. I do find myself dragging out the 300B amp or the KT88 amp for loud listening. Pink Floyd needs the solid bass of the KT88's.

Quote:
I'm wondering if better iron is available more reasonably since the power is so low.
The limiting factor for small OPT's is the inability to pass much power at low frequencies due to core saturation. At 1 watt or less this is not an issue for many small OPT's. I can say that the little Edcor XSE 15-8-5K will sound excellent at this power level and slightly higher. At 1 watt you will get a frequency response of 20 Hz to about 60 KHz. I am listening to a pair as I write this, being driven by a 2 watt per channel "spud" amp.
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Old 8th July 2008, 02:18 PM   #3
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Tom McNally built a mono-block Darling a few months back. His site is http://www.mcnally.cc

I built 2 stereo Darlings in fairly small black Hammond steel chassis 10"x6"x2" and small Triode Electronics output transformers. The amp sounds great and I am listening to it now as I write this post.

Here is a photo of it.

Sal

Click the image to open in full size.
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Old 8th July 2008, 04:22 PM   #4
kevinkr is offline kevinkr  United States
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I mostly use the Hammond aluminum (gray chassis) as they are easier on my tools, and don't have the magnetic issues of steel. (They are more than strong enough to handle the transformer weight of a small amplifier without reinforcement.)

I have built several 45 based SE amplifiers and fixed several dc and older darlings for others.

Personally I prefer 45 based amplifiers, but the Darling is a good sounding amplifier, the 45 a bit better and roughly twice the output power.
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Old 8th July 2008, 04:52 PM   #5
Zap is offline Zap  United States
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Thanks for all of the input so far!

Kevin - Do you have a specific design style/topology or driver that you would recommend for the 45? OPT brand?

I know the 45 has a lot of aura surrounding it and good audio reproduction. I feel that I shouldn't cheap out if I build a 45 amp.
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Old 10th July 2008, 12:51 AM   #6
kevinkr is offline kevinkr  United States
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Take a look on my site for ideas, the design shown there is easily built, but there are many on the web.

I like fixed bias on the output tube, recommend a 5K transformer and 250V plate supply.

Driver stages could be the 6SL7 SRPP I used, a 5842 choke loaded using the MQ EXO-001 100H choke with a green led for cathode bias (2.7V) and a plate voltage of about 150V or the choke could be replaced with the pinkmouse/SY/diyaudio ccs.

You can readily use AC heating on the filaments and I recommend making provisions for measuring the operating point of each output tube. Use a separate center tapped 2.5V filament transformer for each tube. Mil surplus might be the best choice, symmetry is better than for most modern types - this will prevent excessive hum in the output. Use a 10 ohm 1/2W resistor from tap to ground for bias measurement. No 2.5VCT then use a pair of 22 ohm 1 W to create a pseudo tap and hang your 10 ohm from there.

Fixed bias, a misnomer applies a "fixed" (usually adjustable) negative bias to the output tube. This allows you to experiment with running the tube over a variety of currents to see what sounds or measures best (not necessarily coincident I'll add.) and importantly eliminates the need for a critical quality cathode bypass cap.

If you use cathode bias use the WE technique with a good quality film cap from cathode circuit to ground (ASC motor run or equiv.) and a cap from that point to B+ that = 1/mu+1 which is roughly 20% - this provides hum canceling if unregulated plate supply is used. Use a very high quality film cap here.

Quiescent current should not exceed 40mA, but the sweet spot may be lower depending on the operating point chosen. (Vp, Ip, RP) I run somewhere in the mid 30mA range depending on tube.

Leave plenty of space for mods and to prevent coupling from the power transformer into the opts if you build on one chassis.

With careful design and construction you can get 2Wrms @ <5% without distortion canceling, residual noise on the speaker taps of 1mVrms or better and great sound.

Invest in good iron. Electra-Print makes very good small opts for a reasonable price.

I would use the bias tap on a Hammond power transformer to derive the fixed bias if you borrow anything from my old design. I don't recommend the cap based ac divider for reliability reasons, although mine has worked for over 10yrs with the right caps and the amplifiers have been unfailingly reliable - no repairs have ever been required.
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