• Disclaimer: This Vendor's Forum is a paid-for commercial area. Unlike the rest of diyAudio, the Vendor has complete control of what may or may not be posted in this forum. If you wish to discuss technical matters outside the bounds of what is permitted by the Vendor, please use the non-commercial areas of diyAudio to do so.

Amp options for the Betsy's

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi all.

I'm currently testing an amp I designed with the BetsyK. I am using a 25 ohm series resistor.

The interesting thing is that this amp, only capable of 300mA output in class A before clipping and +-4.7V swing, can drive the BetsyK at respectable volume. To my calculations, maximum power RMS into the Betsyk+25 ohm series resistor at 250Hz (point of lowest impedance for the BetsyK) is only 500mW - half a watt.

The amplifier is originally designed as a headphone amp!


My thinking is that requiring so little power (at least in OB mode), it would only take a small amp for normal listening. Chip amps are usually employed in DIY AFAIK, but I'll bet a very simple discrete amp could be developed for the Betsys. Very economic if you ask me...

My amp is class A, and not robust enough to be used for speakers seriously. I'm thinking something like the schematic below would be ideal, with upgraded outputs. I'm not advertising for AKSA, the design is conventional and this was the best example I could think of.


Anyone interested? Group DIY is always fun...

- keantoken
Well, that would work too, but I don't know much about tube design. How many tubes? I'm guessing one VAS and one output? Or is just one needed?

How about... A tube design with knob-variable global negative feedback?

And what about output impedance? Are the Betsys best voltage driven or is some impedance drive optimal?

Addressing this point, if an OPT must be used with a tube amp, we may have to put the OPT in the feedback loop in order to control output impedance. Wrapping the transformer's parasitics inside the feedback loop may require special compensation and will probably decrease feedback performance.

- keantoken
Hi is very simple, two tubes ,triode class A ,single ended ,no feedback.

Current Projects some advanced amp but there is more simple amp for start :http://www.wavelengthaudio.com/bugle.pdf very good

search for 10y-71a-45-46 power tube driver can be 26-01a-31 -a lot more

the iron cost a bit but you can start with edcord ,hammond or james , or used ,old stuff from epay..

I have a supravox 215 s bicone in OB, is similar to betsy k with SE 300b tubes sound wonderfull , a lot better then *borbely* mosfet class A ,no game ! this small pw tubes is the best device on the planet ,period!

Solid state alternative Pass F2 as current amp sound very good with full range , on Nelson site there are interesting paper to read....
Last edited:
Okay, looks like amps aren't a limiting factor here, billions of them good enough...

The main factor to consider for tailoring for the Betsys is output impedance. With what impedances does Betsy sound best?

It depends on the amp I think. With my listening amp I must use 25 ohm series resistance to de-shred the highs. With that in mind, optimal impedance will probably not be constant, IE less resistance at bass, more at highs... Or whatever. Some series resistance is supposed to be beneficial for midrange and above, to make it immune to thermal compression. It is important to be sure increased resistance isn't simply improving the amp's performance rather than the speaker's.

Now no doubt the Betsys sound fine as they are, without any special treatment, but I've been reading about current-drive and impedance considerations occasionally and it seems to me that constant impedance drive is not optimal for any speaker.

So why not test run this theory? I think we want to use impedance tweaks while not disturbing the response enough to need EQ'ing, or maybe some simple first-order filters if necessary.


- keantoken
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.