Remolon

There is a very simple solution that I use for my amplifiers: 1 resistor, equivalent to the headphones impedance, for each channel and a switch. The switch derives the output to the headphones or to ground, but through the resistor. In this way is possible to plug or unplug the headphones without any problem and the amplifier has the same load.

Hola Fernando. Bienvenido

What I do is to put a mute circuit at the output. There is a switch that direct the signal to the headphones or to ground through a resistor (I use 300R).

Thanks for the information. I'll try. I was using http://www.myropcb.com/ The quality and the price are correct but the problem is the shipping method. They use UPS (although the carrier is not UPS but some air companies) and only UPS. UPS adds the shipping cost to the price and the total amount exceeds the custom exempt to be declared. For this reason it is necessary to pay custom duties for the total amount and to UPS for the custom administrative work, and the offer of 20USD for 5 PCBs become around 100USD.

Thanks for the information. I am planning to substitute the 5687 tube output of my Stax amplifier and try the 6S4A, increasing the voltage to 600VDC, with this schematic. The input tubes could be 6DJ8, 12A?7 or a combination of 12A?7, depending of the voltage amplification needed.

Is there any link to the GES Mk2 schematic? Thanks

You are right. Considering the Kevin recommendation and the Headincloud experience the total resistance (R20+VR2) should be less than 1K5, so, a solution is to use the 1K trimmer and a 750R resistor. BTW, in case of using the 1K5 resistor, 1/2W is not enough ( the dissipation with 10+10 mA is 0,6W).

yes for a 1/2W trimmer but not for a 1W such as #652-3250W-1-202

A simple solution could be to choose a 2K trimmer and a 499R resistor, if there is not a minimum safe value for the cathode resistor higher than 499R?

PCBs received. Thanks

@headinclouds: Would be nice to read about the progress of the PCB order. Regards

Thanks Kevin. It is a excellent information.

I believe that not all the valves evolve similarly, and although they are matched at the beginning it will not last for ever. For this reason, I prefer to substitute the need of matching with a procedure of measuring and adjusting a fundamental parameter in the response of the valve. For example, in a power amplifier with a push-pull of EL34 the procedure is to measure the cathode current and to correct it adjusting the grid bias, but what is the procedure for a circuit such as the KGST, if any?

I am waiting for the PCBs to build the KGST but I am considering the option to duplicate, with the appropriate values, the set R20/VR2 for having separate adjustment for each valve and the possibility to correct some valve mismatch.

Any one is OK but I choose 1/8 because its smaller size

For the stepped attenuators I use 1/8W Xicon 270 Serie resistors

I appreciate all these designs and other not in the list, but even more I appreciate the way you make them available to the DIY community and your disposition to help with additional information, answering posts and private communications. Thanks

In three pages there are some more information, such as the correct matching procedure to obtain trusted results and the differences for triodes and pentodes.

Could be interesting to read what Langford-Smith said about matching and the effect of mismatching. http://paleoelectronics.com/RDH4/CHAPTR13.PDF Pag. 580-2

Thanks Kevin. So, choosing this solution and not only one bi-polarized capacitor is mostly for aesthetic reason?

C8, C9 polarity If I understand correctly the PCB, the servo circuit capacitors C8 and C9 are connected this way: -//+ +//-. My question is what is the resulting capacitance and the polarity of C8 + C9? Or there is no equivalence for the set?

@jose It is clear from my post that I have not listen to a KSA-5 but, in any case, I guess that we are talking about different Dynalo. I think that your reference is the one you built with the PCB I gave you at the beginning, when I started the project, and mine is from one a little more evolved, with the same flat response but more powerful, a darker background, more transparent and with a better response to the transients. There is a very simple solution: I can send my Dynalo to the Island, you listen to both, my Dynalo and your KSA-5, an later you send both to the Peninsula, I listen to both for a while and return yours KSA. In this way we can talk having a common reference.

The main question is for the response to the headphones. I was considering to built a RSA-5 but, if the results of the simulation are correct, I do not see any advantage of the KSA-5 over a well constructed Dynalo, with a good PSU for each channel of +/-18VDC and a XLR-4 output with separate ground from the respective PSU, able to provide a excellent response and only a little less power

I am making some simulations of the KSA-5. The maximum output voltage without clipping obtained is a little less than 20Vpp, that is 6W on a 8 Ohm load, and I guess that, with the two pair of mje15030/15031,the amplifier can provide enough current to move (to drive, is better) a efficient loudspeaker. If the load is the 300Ohm of a Sennheiser headphones pair, and adjusting the bias, the output voltage can reach 22.5Vpp, providing a maximum of 210mW to each phone. Is that correct?

You are right. The bias adjustment is missed, as well as other controls and adjustments. It is just a curiosity that shows the interest of people on this design.