Relaxed session

Well, I had the first session in a long time with my EM4 in a somewhat high quality arrangement. I was using the O2 headphone amp and the D-zero DAC from my Macbook, using Fidelia as the source. Short commentary.

Source does matter. Some files were better, and some were worse. But I must say that hearing Bach's Cello suites through this setup was very intimate. In the stillness of the afternoon, and using IEM, I really felt like I was in the room with Janos as he plays the cello suites. On another thought, 1812 Overture was just flat, even on 176.2 kHz source file. I think that complex music just doesn't play well with the setup. Not sure why, but it was just not as inspiring as other kinds of music.

But, it was a refreshing and enjoyable session of one and a half hours.

EL84 PP Idea

Well, I was on the train home and was thinking about the Beyma. I was considering have it passive crossover. Using copper foil inductors and Jantzen caps, it turned out to be around US$350. I was then thinking what if I just built two simple EL84 PP amps instead, and then I can experiment with a cheapie PA active crossover. This was, I can tune them and then once the sweet spot found, I can do a hardwired active crossover. Would that be cheaper?

Anyway, more incoherent thoughts later, see below. 2.6W in SE mode, and thus is Class A PP, it'd be 5.2W. 2nd harmonic should cancel out (mostly) and the 3rd harmonic should sum, but the predicted 3rd harmonic is pretty low. The PSU below using a choke input. It could be a simple, 'cheaper' build. But it could also be quite good. The output transformer is 8k in SE operation.

OB Design

Here is my idea for my convertible open baffle. The overall size is 1,100mm high and either 550 or 600mm in width. It will have a 90mm hardwood return frame that may have bracing and sound damping applied. The back will allow the addition of a boot or shroud that can make it a U frame, closed or ported box. The main issues will be how to make it rigid enough for the closed and ported box.

The lower driver will be an Eminence Alpha 15A. Simply because I have a pair. They could be changed in the future, but for the moment, that at it. The main speaker panels will be removable to allow me to change different drivers. I am not sure whether to have a separate panel for a super tweeter,or to have that sitting on the top.

But if this does work, I will have a very modifiable and tenable OB speaker that will fit my Fostex F207e, Lowther DX3, Sabe Green cones and potentially also Visaton B200, Supravox 6" or 8" Alnico, PHY or anything else I can get my hands on.

This, my Beyma Coax ported box and the Jordan TL will form my main speakers.

Oarta output transformer

It looks like the Lundahl LL1623 rated at 90mA would be the best choice for this amp. The amorphous core price is less than half of the Tamura, but more importantly, it is available. The interstage transformer would be the next choice. This would probably be the most important choice, given the different voltage swings that needs to be accommodated by the different tubes.

I think at this point, it would be important to at least try to limit the range of tubes to be used. This has to be correlated with my current stash of tubes. In this, I would say that there are four families that I would like to run:

1. 45, 145, 245 and 345. I have a number of pairs and have been universally hailed as an excellent audio tube.
2. F2a. I have three pairs of these long life tubes.
3. KT88/EL34 family. I have two quartets of the KT88 and EL 34.
4. 300B. I have one pair. 

Looking at the above, it would make most sense that I provide more effort towards the first three families given the volume of tubes on hand. With the 300B narrowed out, the voltage swings required are now limited to 30 Vpp to 60 Vpp, a little more sensible. This would mean that a tube with a gain of 20 would be plenty. At this point the 5687 and C3g seems the right direction. If I use a higher gain tube, I can use a step down interstage, which should be a good think overall. The other option, is to allow a different tap on the interstage to be used to connect to the grid of the output tube, which can select between say 1:1 and 2:1. But this may affect the performance and bandwidth of the interstage.

More questions... 

Oarta Output Transformer

It seems that the Tamura F5003 is in limited supply, and may not be available. So I may have to revert to another brand of amorphous core. Maybe Lundahl?

Revised Beyma Box

I have been playing around with the box model for the Beyma co-ax, and since then, I have made the following changes. 

1. 300 l vented box - a nominally 1.2 H x 0.55 W x 0.6 D box.
2. Vent tuned at 30 Hz.
3. HP at 20 Hz, 2nd order.
4. LP at 700 Hz, 4th order.
5. No Linkwitz transformer.
6. Source voltage at 14 Vrms, which is around 25 W amp.
7. Disabled baffle step correction.

At this condition, I was able to get the following predicted response. Note that the driver excursion is limited to below 4mm. At 25W power, it is giving around 100 dB, with f-3 at around 27 Hz. Fairly decent result, with less tinkering with EQs. Bigger box, but that may have to be the sacrifice. 

Beyma Ported Cab

To design a simple box for my Beyma 15XA38Nd, I have employed Basta. Using a 200l ported box with two 75mm diameter 147mm length ports (tuned to 25 Hz). The box is also fairly standard at around 1m in height, 0.5m in width and 0.4m in depth. Something easy to construct.

My speaker box model also includes a Linkwitz Transform at 20 Hz along with a 4th order low pass at 1,800 Hz in tandem with a 2nd order high pass at 15 Hz produces a near flat frequency as model between 30 Hz to 700 Hz. With -3dB points around 20 Hz and 1 kHz. See below.

Crossover at 1.8 kHz and maximised bass response

There is quite a lot of filtering in there to achieve this. The 1.8 kHz crossover was set from the standard Beyma passive crossover (2nd order) tuned to that frequency. I think that 1.8 kHz seems a bit high for a 15" driver, so if I set the model to 1 kHz instead, it gives a much more 'flat' predicted response. Note also that the Xmax of this driver is 4.0mm and according to the model we hit 3mm at around 20 Hz with 1W of input. I may have to tune the high pass from 15 Hz to something higher to limit the response. The below shows using a 4th order HP at 20 Hz, which limits the Xmax to under 2mm at 20 Hz. 

Crossover at 1 kHz and slightly reduce bass response to limit Xmax.

Given my listening room is only 5x4m in size, I think that a flat response down to 30 Hz may well be plenty enough. My question now is how to experiment with the various cross-over points? The easiest solution would be to employ a PA style crossover, something cheap from Behringer or similar. Which would allow me to test and tweak to my heart's content. 

Another option would be to make the box closed, and with the same settings as the second scenario above. The response is still pretty decent. 

Closed box with same parameters as above.

To be honest, if the speaker performs anywhere near what the model shows, it'd be great. Even if I were to use a closed box or vented with the standard passive 2nd order filter, the response is still pretty decent. See the two simulations below.

Passive crossover ported box, no EQ.

Passive crossover closed box, no EQ.

Oarta PSU

One interesting point is that if the PSU is cap loaded with a 10uF, at 200mA, it will give around 380V. But if I remove the cap, and have it choke loaded (using the LL1673), at at loads of around 100mA to 150mA, it will give voltages of 260-280V. Perfect for 2A3 and 45. So in this case, if I can provide switchable PSU filter, I can further extend the range of usable tubes, rather than using resistors to drop voltages.

Oarta - Some Design Images

PSU Design.

KT88. 400R cathode 

300B. 740R cathode

KT66. 300R cathode.

EL34. 400R cathode - can share the same plate as KT88.

6550. 400R cathode - can share the same plate as KT88.

The Oarta - One Amp to Rule Them All

It has certainly been a quite a long time since I have done ANY DIY audio stuff. I blame this due to my relocation to Hong Kong for the last 18 months and having young kids. There just ins't time. Well, with the imminence of return back home on the horizon, I have started to dream up what I will finish up. In this long time away, I have realised that I like variety. But I don't want to build too many amps. So, I have taken a few ideas, based around a SET and merged it into one amp. Which I call - Oarta, or One Amp to Rule Them All, not exactly creative.

The basic premise of this amp is that the PSU will allow the supply of around 390V at 200mA max, with heater windings two sets of heater windings, 2x 6.3V at 2A and 1 or 2 5.0V at 2. This should allow me to use a large range of tubes that I have on hand: F2a, 300B, KT88 and EL34. And also open up a whole range of other tubes that can run at those voltages.

The second part is a universal output transformer. I have chosen a 5k tap, but may move to a 3.5k for more power. For this amp, it will be no compromise, so a amorphous Tamura F5003 or F5002 will be on order. The driver stage will be a medium mu (think around 20) that is interstage connected to the output. To accommodate the wide range of output tubes, I will probably want a higher gain stage, so C3g triode connected may well be the final choice.

The trick here is to allow the switching of the output tube. To this, I am thinking of an adaptor plate that allows the socket and cathode resistor to the changed. The bypass resistor will then be mounted on the chassis. In this way, I can have a large film cap that isn't changed with each socket. On each adaptor plate will be 5 connections; grid, plate, cathode, heater + and heater -. I need to find safe and reliable connectors that allow me to change.

The final trick is have a 6.3v and 5.0v winding, and finding a method to accommodate a range of heater suppliers. For the indirectly heated 6.3v tubes, this is no issue, but with the DHT tubes, like the 300B, PX25, etc., this will need some more work. There may need to be an allowance for something like the Tentlabs filament supply, that can run from the 6.3v to power a 5.0v and run from the 5.0v to power a 4.0v tube.

If this can be made to work, I have an amp, Oarta, that will allow me to change tubes, without having to change anything else. This will allow me to not skimp on the rest of the components, knowing that it will be universally applicable. At the moment, my current thoughts for the design are:

PSU.
Amplimo 7N1474 Transformer (here - I may need to supplement with additional heater taps)
2x 6AU4GT diodes
Cap input 10uF ASC Blue line
Two LC stages with Lundahl LL1673 200mA (here) each with ASC 50uF.
I am thinking about a separate LC stage (15H with 40uf) for each channel and an additional LC stage for the driver.

The main issue with the PSU is finding space for all the ASC oil caps and the chokes.

This will give me around 386V of B+ at 190mA current draw (simulating with PSU Designer). At this B+, the following would be the power output with different tubes (from SE Amp CAD) using F5003 all running at 80mA per tube.

300B: 7.6W with 120 Vpp
KT88:  6.8W with 62 Vpp
EL34: 8.8W with 60 Vpp
KT66: 4.5W with 46 Vpp
F2A: 5.8 W with 32 Vpp (by hand)

And I would think that this would also work with tubes like GU50, PX25 and some of the other more rare European or exotic NOS tubes. And if I don't mind burning off voltage as heat, it could be used on 2A3, 45,  AD1, etc. - again, options. Beyond these, there are still a number of issues to work through, but in my mind, the concept already has been resolved. I am now thinking about cathode bias for the driver, and most likely will head down the LED route. Given the C3g triode seems to work well with around 2.0-2.5v bias. 170V at the plate, 2.5v bias 15mA. Nice and simple.

The main course of action now, is to determine how best to build the adaptor plate, this will make or break the design. It must be safe, easy and high quality to preserve the rest of the amp design. And the last question is how to deal with the different voltage swings required by the various tubes. Which differ by a factor of 4. I do not want to simply waste it by using an attenuator, and the first thought was to have taps on the interstage, which would allow me to choose the right ratio, 1:1, 2:1 or 4:1. This I think is possible with the Sowter 9525. But I probably want to avoid changing the IT ratios as that will change the frequency response characteristics.

Or I could just vary my source output - say between +0 dBV, +6 dBV and +12 dBV. (Which incidentally is possible with the Wadia 121 DAC that I am considering, see manual). Or, I could just accept it, and have to set the volume levels differently for each tube. This though, needs more thought and consideration.