Darling SE amplifier

 

Single-ended tube amplifier with two 1626 tubes in parallel per channel

Used in my living room

 

Associated equipment:                                                                           Last updated: December 2, 2005

 

Sony ST-S370 FM Tuner

Marantz CD-80 CD Player

Homemade Phono Preamp (with JFETs)

Dual 1019 Turntable with Empire 999VE MM cartridge (left from my dad)

Jericho horn loudspeakers

 

 

Six JAN (joined army navy) tubes, after some 40 years of rest,  finally make music…

 

I started this project with the idea to build a simple single-ended tube amplifier. I didn't have any experience with the construction of tube amps so I was looking for a beginner's project. Such a project should be simple (for easy debugging) and it should also be cheap. Nobody likes to blow up a component of 300 dollars by a small lapse of attention...

 

As far as simplicity is concerned, nothing beats the "Darling" series of amplifiers which are described on the website of New Jersey resident Bob Danielak 1. They employ power triodes type 1626 which can be obtained cheaply from various sources (I bought several for 3 US dollars apiece). Old readers of this page may remember that the 1626 was employed in various transmitters for the American airforce, e.g. the AN/ARC-5 series (BC-457, 458, 459 and 696). Since they were used in military airplanes, 1626s are rugged valves which can stand shocks, vibration and electrical insults. Once I had excessive power dissipation in two 1626s because of a wiring error. The plates became fiery red and blazingly hot and remained in this condition for two minutes before I discovered my mistake. Yet these tubes still sound and measure OK. Apparently, Darling amplifiers make excellent projects for dummies like me!

 

I decided to make Jeremy Epstein’s DC-coupled variant of the Darling amp with two parallelled 1626s per channel 2. The schematic is shown below:

 

 

 

 

 

For my first try, I followed Jeremy's schematic to the letter, although I used a  GZ34 rectifier in stead of a 5U4G. The Hammond 125E output transformers, 1.5 H choke plus the 8532 and 1626 tubes were from Antique Electronic Supply in Tempe, AZ. A NOS power transformer (Prova) supplied 2 x 345, 5 and 6.3 Volts. I acquired this tranny and the GZ34 rectifier from Frits Meuris Electronics (Sittard, Holland). The 12.6 Volts for the 1626 heaters came from an obscure transformer from my junkbox which I bought for one Dutch guilder from Conrad Electronic.

 

I built the amp in the old style, hardwired on an aluminum chassis in a wooden frame with all tubes, chokes and trannies on top and all electrical wires below. Special parts were not used. I took what was in my junkbox (carbon resistors and styroflex capacitors from the former DDR,  electrolytics from Czechia, wirewound resistors from Vitrohm in Germany). After I had corrected my wiring error, all voltages were within a few percent of Jeremy's values and 1 kHz square waves looked OK. No trace of microphonics. So on for the listening tests!

 

After a burn-in period of a few hours, I listened to live recordings from twentieth-century classical music (Schönberg Ensemble), using the multimedia speakers which are described elsewhere on this website. The amp sounded wonderful, as if you were in a very good seat in the concert hall. I was really excited about the "SE magic". But after a while I became disappointed. The amp sounded excellent with some programme material (chamber music, small combos), but it didn't sound good at all with large orchestra, grand piano, church organ, or big drums. The presentation seemed then "anaemic". Especially piano recordings sounded terrible. Measurements indicated what was wrong:

 

 

 

The frequency response of this version of the amplifier was 75Hz...14 kHz (-3 dB). With orchestral music and grand piano, I was missing the lower two octaves. For Jeremy this didn't cause problems, for he employs his own Darling as the treble amp in a bi-amped system. But as a stand-alone amp, the results were not satisfactory.

 

Jeremy thought the cheap Hammond transformers were limiting the bandwidth. So I decided to try more expensive output trannies. I replaced the Hammonds with Lundahl LL1664s from Sweden which I purchased from Aqua Blue in Belgium. The (almost fivefold) difference in price certainly resulted in improved specs (see below).

 

 

 

With the Lundahls, the frequency response was flat from 8 Hz...55 kHz (-3 dB). This was a major improvement! Grand piano now had weight and sounded like the real thing. Sibilants were somewhat emphasized in the Hammond 125E version, but sounded natural with the Lundahl LL1664.

 

So I was very happy for a while. But the perfectionist bug hit me once again. IMHO, the amp still had a major flaw. There was audible hum in the output. With the multimedia speakers, the hum was not objectionable during the reproduction of music. But in silent passages, it became annoying. Additional measurements indicated that the signal-to-hum ratio was only -55 dB with shorted inputs!

 

First I thought that I had made a grounding error, but this proved not to be the case. Then I thought: maybe I should use DC heating. However, there was no improvement when I hooked up the 8532 heaters to a DC power supply. I noticed that the hum had a frequency of 100 Hz rather than 50 Hz. I concluded that it originated from the B+ supply.

 

After running some simulations on Ben Duncan’s PSU designer program, I decided to modify the B+ circuit. After the GZ34, I maintained the 47 uF electrolytic capacitor.  The stereo channels are connected to this cap via individual LC filters (10 H chokes and 200 µF/500V electrolytics). This modification resulted in a 29 dB improvement of the signal-to-noise ratio. The capacitive load to the GZ34 remains within safe limits. And hum and noise are now completely inaudible.

 

Specifications of our modified Darling:

 

Output power: 2 x 1.5 Watts in 8 Ohms

Voltage gain: 8.6 x

Input sensitivity: 400 mV

Frequency response: 8 Hz...55 kHz (-3 dB)

Signal-to-noise ratio (guesstimated): -84 dB (related to 1.5 W output)

 

This is an excellent amp. It sounds lively, agile, and natural. Makes you tap your feet and sing along with the music.

 

Verdict after two years of use: We listen daily to this amp on the large Jericho horns, so you can guess that we are pleased with its sound. However, the NOS power transformer (Prova) suddenly broke down (catastrophic damage to the insulation of the B+ windings) and we had to resurrect our killed Darling, using a custom-made transformer (purchased from Kent Electronics). We now used a metal chassis (Hammond) and we replaced the 1000µF 250V electrolytics with 47µF 400V polypropylene capacitors (Audyn-Cap, Intertechnik). This slightly improved the sonics. With the electrolytics, the sound could occasionally be somewhat "hard", with the foil caps this never happens, not even in difficult passages. This tube amp sounds sweeter than any of my solid-state products. Finally, we added volume controls (100k log). Now, the amp can be driven directly by line-level sources (CD players, FM tuners). A preamp is no longer necessary.

 

Finally, a picture:

 

 

 

Darling (front view). A protective cover was mounted over the transformer after the testing phase.

 

Questions, comments: aren.van.waarde@hetnet.nl

Tip: Use only my initials if you want to reach me

 

 

References

 

1 Bob Danielak, “Darling” and “DC Darling” SE 1626 amps”, http://www.geocities.com/bobdanielak/darling.html

 

2 Jeremy Epstein, “My experiments with Bob Danielak’s ‘darling’ 1626 amplifiers”, http://home.earthlink.net/~jeremyepstein/darling.html

 

 

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