Remolon

Schematic posted on June 2010: http://www.diyaudio.com/forums/solid-state/109618-krell-ksa-5-a-4.html

Considering that the power supply will be +/-350VDC, do you think that for R26 and RV1 would be adequate to use similar values that the used on the original (HeadWize, 2001) KGSS. That is, 3K48 for R26 and 2K0 for RV1? I have an other question. For using the amplifier with a unbalanced (just regular RCA) input, and following the recommendation, the +input will be connected to the potentiometer output and -input to ground. The R48 resistor will be in parallel with the potentiometer resistor connected to ground and the resulting value for the +input will be much lower than 500K. In these case, would be recommended to use a lower resistor value for R49? If yes, do you consider appropriate a value of around 7K5 for R49 with a potentiometer of 50K at the input?

Thanks. I have finished a prototype just to check if everything is correct, before ordering PCBs, and no problema. With a input of 2x300VAC, the output are exactly +350.0, -350.0 and 580.0 volts

The disadvantage of moving the connection of the trimmer to the V+ output is that C1, C2 and C3 will not discharge through RVB. A drain resistor, RD, has been added to solve the problem.

The bias Zener are 3x200volts just in case the voltage get to high for the specified 580VDC. The bias will be regulated with the trimmer RVB. Your comment make me to consider the option of moving the connection of the trimmer to the V+ output, more stable. Thanks

I plan to use this PSU to obtain +/-350VDC and the 580VDC for the bias. The Zener DZ and the resistor R3 define the current which pass through R3, R4 and R5, and, obviously, the output voltage.

Thanks Kevin. The substitution is the option I had because some transistors are almost impossible to obtain.

I agree that every DIY project is a opportunity to learn, and the KGSSHV is an excellent example. I am mounting one of the Amp PCBs provided by Lil Knight. Because of the lack of some parts, I made some substitution (probably some mistakes). So, I am using two BC550 glued with cyanocrylate to replace the LSK389, for Q20 I am using BC550 rotated 180º, BC556 for Q17 and Q18, and KSA1156 for Q15 and Q16. Would be possible to use KSA1156, with the correct pin connection, for Q5 and Q6, if the power supply do not pass +/-350VDC?

This is the Data Sheet that I have http://www.sm0vpo.com/_pdf/2SA/2SA1968.PDF

You are right. The current source need a PNP transistor but the 2SA1968 is a NPN one, and I was looking for an alternative to the 2SA1968 and not for a high voltage PNP, according to the schematic. Sorry.

Thanks Kevin for the new schematic. I do not have any 2SA1968. So, I am considering the option of using 2SC4686 for the current source (Q5, Q6). Do you think that it is a good alternative or it is better to go with the optional current source (IXTP01N100D for Q7, Q8)?

It is D1, output stage

LSK389/PMBFJ620 substitute. The GKSS specifies for this function a 2SK389 double FET, as is used for the Dynalo. For my Dynalo, I substituted it for two BJT, first with the ksc1815 and later, with a different PCB, with the BC550. Both cases worked. I do not have the ability for soldering the PMBFJ620 and I do not have the LSK389 or the 2SK170. So, I am considering the possibility of using two BJT (with the base at the central pin) in its place: MPSW06 rotated 180º or BC550. I will appreciate any comments or experience on such a substitution.

Received today. Thanks

Corrected

Here are some pictures of the amplifier for my Stax SR-407. It is a hybrid construction, the power supply and the phase splitter/differential amplifier are PCB mounted and the output stage is point to point connected. The pictures correspond to the initial mounting but the current assembly has no changed, simply there are less holes.

A minor observation. My experience with the pro bias Zener is that the actual value is a little lower than the nominal value. I am using three 200 Volt (Mouser 625-Z4KE200A-E3/F3) to obtain 582 Volt.

I am using TL051, after a comparative of several OpAmp for servo by Douglas Self.

Eppur si muove I have already finished and tested the amplifier. Built with two 5687WA, for the output push-pull, three ECC88, for the differential amplifier, and fixed bias, around -8V. It works very well. I am using it with a pair of SR-407, and with the potentiometer at 11 the SPL is enough for my listening habits.

It is what I had calculated, 320Vpp, with the tubes biases to -10v and a plate load of 30K, as you can see in my previous post. That is 113Vrms, 101dB for the SR-007, with much lower THD that the proposed Stax Model “A”. The bias to -150v is a big mistake, I think that the 6SN7-GT should be biases between -10v and -12v, -8v if not much output voltage is needed, but in no case beyond -14v.

I would appreciate numerical values specific for this case. The datasheet specifies a cathode average current of 20mA and a maximum of 70mA, and the calculated values with a plate resistor of 30K and a voltage supply of 600V are, obviously, even lower than the average of 20mA at any point of operation. Probably there are other reasons I am not aware of them. Any indication is welcome.

Sorry for the prophet, but I consider the Stax Model "A" as a more qualified candidate to be “the worst electrostatic headphone amplifier ever...” displacing the one I had proposed: This is a generic circuit which accepts different valve combination. The choose of the right combination between 6dj8, 12at7, 12au7, 12ax7 and others for the input differential amplifier will provide an adequate gain. In my case, I am using 6dj8 for V1, V2 and V3 to have a gain of 25dB. As I said, I am making the first attempt with the 5687WA for the output. If it works as expected, I will considered other candidates for the output tubes depending of what is needed, more amplitude or more current. If the election is the 6SN7-GT and a power supply (B++) of 600V, I would not use the values on theModel "A" schematic. I think it is better to work with other load resistor and bias; as a starting point, this one: Rp4 = Rp5 = 30K ( referred to my schematic) and Vg = -10V, fixed or with Rk4 = Rk5 = 1K0. Theoretically, Vpp = 320 (450-130), with less distortion and a little more that 100dB for the SR.007 (off course, the 6SN7 admits much higher plate voltage but I would prefer not to exceed 450V). My choice would be for fixed bias, separately adjusted for each valve, and Rk4 = Rk5 = 10R for an easier control.

Does this graph represent the operation of the schematic? Is the operation point Va=400V Vg=-18V and 4.5mA? What is the maximun output Vpp, and with how much distortion?

But how many are needed to obtain eight matched quad? It is not a fair comparison, the two Dynalo have different setup: both have two mono PCB for audio but the one with ksa1015/ksc1815 has a power supply for each channel with separated grounds and a four pins DIN headphone output; and the other is a conventional setup, one PSU and a 3-ways output jack with a common ground.

Me too, I have another Dynalo with BC550/560 but not because I think that the Dynalo accepts everything but because of the similitude of the BJTs. BTW, Could the BC550 be a substitute for the 2SC1815?