Pros and cons of different step-up manufacturesrs and VA ratings

[altie]

I recently came into a set of Janszen ESL panels along with their associated circa-1960 step up and bias circuitry, apparently from a Z-134A speaker set. Naturally, even though they sound great, I'm already thinking about what could be done better, and am looking at power toroids to replace the EI-core step up transformers they came with.

They'll be used in a hybrid application. Right now, for apples-to-apples comparison with other tweeters I've been demoing, they're passively crossed at 2KHz 2nd order. I need to measure, but it sounds like they may realistically work as low as 350Hz or so, and if that's the case, I'm thinking something like 500Hz 4th order active might be a reasonable compromise of accuracy and dynamic range.

So let's say I have a 500Hz crossover hybrid system. I've got two choices I need to make then if I'm going to upgrade the step up transformers:

1) AnTek, Triad or other? Maybe these guys? http://www.newark.com/multicomp/vtx-146-050-106/transformer-50va-2-x-6v/dp/33P8755
2) What VA rating? Browsing previous threads leads to the impression that higher is required for greater bass, but if that's not a concern in my hybrid system, I might actually want to go as small as possible in order to push the system's upper low pass filter higher... although given my estimate that pairs of these panels would make for about 50pF each, some back of the envelope calculations suggest that that might already be up around 300KHz anyway. Does that sound right?

 

[bolserst]

Concerning the crossover point:
You mention needing to measure, and that sounds like a good idea before deciding on the crossover point. The JansZen panels have diaphragm resonance of around 1kHz. They usually have a stiff felt pad on the back of the panels to damp the resonance, but you will still need to consider the boost at resonance when designing the crossover. If you plan to play these at high SPL, you will be better off crossing above resonance in the 1.5Khz to 1.8Khz range to avoid the harsh sound generated at resonance when driven hard. To experience this, remove the passive crossover and slowly sweep with a sine wave around 1kHz till you find the resonance and try at increasing volume levels until your ears tell you to stop.

Concerning transformers:
For ESL step-up use, the VA rating of a toroidal power transformer is not of concern. It is voltage handling capability before core saturation that is important and all power transformers with the same secondary voltage(ie 2x6V) will have the same voltage handling capability.
More details on this here: DIY bass transformer for ESLs ?

Measurements of 15VA VTX transformers: (no problem at all for hybrid use crossing above 250Hz)
Response and power handling: About to take the ESL plunge
Distortion: About to take the ESL plunge

I have measured Antek and VTX transformers and both had low distortion and would not have a problem driving your set of 4 JansZen panels. As you mentioned the capacitance of the JansZen panels are quite low, much less than the winding capacitance of the transformer, so high frequency bandwidth won’t be an issue. In your particular case the most cost-effective way to get the OEM step-up ratio of about 50 would be to use 2 of the Antek AN-0105 with both the 5V and 115Volt windings paralleled (ie 115/5 = 23, 23*2 = 46) or the Antek AN-0209 with the 9V windings in parallel and the 115Volt windings in series. (ie 230/9 = 25.5, 25.5*2 = 51)

The VTX transformer you mentioned (VTX-146-050-106) will work, but they are more expensive and may have higher step-up ratio than you need/want…75:1. There are some VTX 9V transformers that would get you closer to the 50:1 ratio of the OEM transformers but they appear to be all out of stock at the moment.

 

[altie]

Thanks. Yeah, I've got the equipment to do measurements, so I'm overdue for bypassing the crossover and measuring their full range response. I just don't want to unhook them because I like listening to them too much

Shortly before your response, I went ahead and ordered a couple of these guys: 6501K-P1S2 TALEMA - Transformer: toroidal | TME - Electronic components. I based that on generally positive feedback on Talema transformers around here (not that anyone seems to have complaints about Antek or Triad, much less evidence of audible differences), and just liking the look of the sealed enclosures. Maybe when I get them I can hook them up to my sound card and measure frequency response. Will ARTA or something like that help me in figuring out leakage inductance and self-capacitance?

 

[bolserst]

Those Talemas should work great, and look good doing it.
Yes, ARTA can be used to measure the transformer frequency response and the parasitics(leakage inductance and self/winding capacitance). Have you measured impedance before with LIMP? That will be the easiest way to determine the parasitics. LIMP is ARTA's impedance measurement tool.

 

[altie]

Have not. Thanks for the tip. I actually haven't used ARTA at all yet, just REW and occasionally audacity for any kind of audio measurements, but I had seen it come up before so I thought I'd ask. I'll check out LIMP, see if I can figure it out and come back for help if needed.

 

[altie]

Bummer... I'm finally getting back to this project and realized that my Talema sealed transformers have neither center taps nor dual sets of wires on the 230V side. Anything I can do with these to make a bias supply ground?

 

[Calvin]

Hi,

You can connect the 9V windings of two trannies in parallel and their 230V windings in series.
Requires of course two trannies per panel.
I prefer the 2-tranny solution, since it results in the highest (internal) flashover limits.
The flashover treshold (typically specced at 4 or 5 kV rms) applies to primary-secondary.
Inter-primary or inter-secondary values can be considerably lower..
Also potting increases the capacitance values .... unpotted trannies may be a better choice in this regard .... besides the single prim, single sec, unpotted types are typically the cheapest tformer versions availabe.

jauu
Calvin

 

[bolserst]

…my Talema sealed transformers have neither center taps nor dual sets of wires on the 230V side. Anything I can do with these to make a bias supply ground?

As Calvin mentioned, most people use 2 in push-pull to provide an easier center tap and double the step-up ratio. I must admit, I thought that was what you had intended to do. This configuration will give you a step-up ratio of about 1:50. The Talema data sheet does not specifically show the phasing of the different windings with the usual dot notation. The attached schematic assumes the dots are at the top of each winding (ie positive phase entering at pin 6 of the primary will produce positive phase output at pins 7 and 11 of the secondaries.)

If at some point you want to use a transformer without a center tap and not in a 2 transformer push-pull configuration, you can use the Acoustat method of adding a virtual center tap using resistors.
No CT on Transformer for Bias return

…I actually haven't used ARTA at all yet, just REW…

If you already have REW up and running, it will measure impedance as well…so you don't have to switch to ARTA unless you want to for other reasons. Some links on the topic of REW impedance measurement...
Impedance Measurement
UMIK - linux/Java install - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com
https://www.avsforum.com/forum/155-...y-impedance-measurement-jig.html#post20557144

 

[altie]

You know, I guess I did intend to do this in series/parallel since that's been talked about a lot, but it must have slipped my mind when ordering and then when they got here I assumed I had what I needed for both channels. I'll probably just try hooking them up on one channel to do some A/B measurements and see what I find. Got some issues with my measurement gear to sort out before then though... measurement mic is not sounding good when recording voice as a sanity check and is showing a noise floor of 1-2%THD I think. Hopefully soon though...