I devoted the majority of my audio life to single-ended DHT amplifiers. I don’t regret the path, however, I do not want a polarised perspective on amplifiers. I do like very much a good push-pull amp, though. Over time, I stumble across poor implementations or excessive use of gNFB. Local feedback (lFB) designs well implemented are music to my ears.
I played around with SPUD amps over the years, with happy results. One of them I loved over the breadboard is the 6e5p in push-pull. I will definitely build a definitive version of it. Here is the concept overall:
The input transformers is step up. It will need 1:4+4 ideally. My friend Dorin from DVB Transformers has a great device which I reviewed before and would be ideal for this amplifier. To simplify the power supplies, I used only one, which provides a bit of a strain into the source followers SF1 and SF2. If you have a good driver, you can omit them, I wouldn’t. They take care of the miller capacitance which is worsened by the step up transformer. It’s just a pair of PCBs, not big deal. RK1 is an arrangement with a pot to balance the bias. C1 is in ultrapath. I used an Oil 40uF one. T1 was a Lundahl 8K8:8Ω PP OPT.
The CCS provides a stable bias current into RBIAS1 which sets the bias voltage.
Here is the operating point I used:
The 6e5p is biased at about 40-43mA and 240V. It needs 5V bias so a cathode resistor of 58Ω should do. For 4W output in A1, you need 4Vrms, which only with a 1:2+2 step up should do. You can get about 5W in A2 or a tad more.
I loved the sound of the 6e5p as output stage (as much as I do as driver).
There are multiple variances to explore indeed
I have a pair of Toroidy PP OPTs with CFB, so would be interested in running them in tetrode instead.
Hey Ale, for a 1:4+4 input look for the Sowter 9062. With those fet followers it is ideal to use and you won’t need any zobel. It’s 600R nominal input impedance. However I prefer its brother 9063 which is 1:2+2 with much better FR up to 90 KHz (-3 dB) despite the splitting duty. Both have mumetal core (that’s why the follower is a good thing to have), perfect balance and very good headroom with insignificant distortion over the full range. Something like THD better than -90 dB at 1 Vrms input. Cheers
Hi Paolo. Yes, I’ve used the 9062i for some time. Just now I tested it again (coincidentally) with Morgan Jones today at home and found a dissapointing response on the 1:8 setup. It worked fine on the 1:4 wiring mode. I will put it back on my system and listen to it.
Cheers,
Hope you’re enjoying the summer back in Italy?
Ale
Unfortunately the FR is not great in 1:8 mode. In 1:4 it gets to about 35 KHz (-3dB). The 9063 instead is much better it gets to 90 KHz (-3 dB) in 1:2+2 and even better in SE mode 1:2 step-up. Of course this is a typical value and a bit application dependent (in particular on the input capacitance of the driven stage). Also another input transformer worth a look is the new 1475 which is high impedance at 10K and ratio 1:1+1. This replaces the previous 8920 with the great improvement in terms of FR and it doesn’t need any zobel from very low to 10K source impedance for most applications. This one too has mu-metal core. High permeability cores make a big difference at low signal level. Much better than any amorphous or silicon-steel……