Modified Follower-99 With HF Transistors

[VladimirK]

I have put into service pure class A amp based on the following schematics. Assembled point-to-point, sound is amasing. Gives impression of a "direct wire". I compare it with SE 300B based amp. Similar sound transparency, but overall is better.
I understand real problem with getting RD100HHF1 transistors, but I gues other good ones will go.

[Samuel Jayaraj]

Can VT3 be substituted with 2SK170BL? And VT1/VT2 with equivalents from Fairchild? I am asking since I may be able to try these with substitute parts. Thanks.

[VladimirK]

Originally Posted by Samuel Jayaraj

Can VT3 be substituted with 2SK170BL? And VT1/VT2 with equivalents from Fairchild? I am asking since I may be able to try these with substitute parts. Thanks.

Yes, VT3 can be 2SK170BL or 2SK170GR, and VT1, VT2 - any decent MOSFETs, 44N08 or K1529, K1530.
Most of all sound affected by the output cap design (I used Jamicon WG series, but Panasonic FM 1000uF could be even better), and big polypropylene shunt is obligatory.
Be shure, sound will pleasantly surprise you.
One possible difficulty will be a good preamp, tube one with no less than 20dB amplification will fit.
My solution for preamp I posted at Pass forum
The 50W version of Zen9

[Samuel Jayaraj]

Sorry I forgot to ask earlier but how tight and articulate is the bass; mids and highs should be as good as tubes, from your report.

[VladimirK]

Originally Posted by Samuel Jayaraj

Sorry I forgot to ask earlier but how tight and articulate is the bass; mids and highs should be as good as tubes, from your report.

In my system, with the preamp indicated above (NO NFB single-ended stage) and the present Follower, as a result I have NO NFB system with output impedance nearly 0,2 Ohms. Believe, 0,2 without NFB makes much better bass than 0,2 Ohms emulated by NFB, there is some specific artefacts induced by the speaker back EMF. For comparison, impedance of my High-End Stereovox speaker cable is near 0,4 Ohms.
With 50W Zen9 bass is also good with Aleph CS being active, but without it, the highs are close (almost) to the Follower, but the bass of Follower is noticably better.

[VladimirK]

Here are some fotos of the Follower, point-to-point soldering

[VladimirK]

I have measured frequency dependent output impedance of this follower (mOhms on the vertical axis, and Log10 F(Hz) on the horizontal), and also of two modifications of 50W Zen9.
Curve 1 - present follower
Curve 2 - 50W Zen9 with increased NFB, voltage amplification near factor 2, without current source modulation
Curve 3 - 50W Zen9 with standard amplification factor 5, current source modulation is active
Variants 1 and 2 are close in middles and highs reproduction, but the bass is the best with follower

[Samuel Jayaraj]

Are V1-V3 9v zener diodes? And are HL1-HL6 LEDs (what colour, voltage drop)? 1KVA transformer! What concessions can be made in this costly component? It will be nice if you can make a full bill of materials, indicating modern alternatives. Thanks.

[Jay]

Wow, what a wonderful mosfet the RD100 is! But I can't see circuit improvement that will solve the 99-Follower's problem if ordinary IRF mosfet is used.

How come this RD100 is not popular? (at least unknown to me)

[VladimirK]

Originally Posted by Samuel Jayaraj

Are V1-V3 9v zener diodes? And are HL1-HL6 LEDs (what colour, voltage drop)? 1KVA transformer! What concessions can be made in this costly component? It will be nice if you can make a full bill of materials, indicating modern alternatives. Thanks.

V1-V3 are the 9V batteries used for home appliance, they produced so call battery bias, that is used sometime in tube schematics and also in some high-end solid-state amplifies. It produces quietest bias for the sensitive points, among all possible. An alternative to it will be usual resistor divider with an electrolytic cap, which I try to avoid everywere if possible.
HL1-HL6 are the red light diodes, with near 1,8V voltage drop on each. They represent simplest, but extremely fast and low noise shunt power supply for VT3-VT4. As yuo not, I do not use here electrolytics as well.
Using VT3 instead of resistor, makes CCS composed by VT2-VT4 more close to an ideal one.
1KVA transformer looks like a joke, but I have not managed to avoid electro-mechanical trafo bussing with smaller ones, since my aim was an absolutely pure backgroung. If you manage to avoid buzzing under 4A load with smaller trafo, that will be your benefit.