Yet another EQP-1A

[Wombaticus]

Hi All,

I've been working on an EQP-1A build intended as a companion piece to the LA-2A-inspired build I posted about here several months ago. I used a new layout for this, and incorporated (yes, I know it's probably overkill) DC heaters for the tube filaments. At any rate, I'm very pleased with the way the layout has came together -- it's rather different from (though intentionally not as compact as) Cayacosta's layout on RecProAudio. At any rate, if you have an interest in this circuit, keep an eye on my blog... with luck I'll have some sweeps and sample posted in a few days. For now, there's some gutshots of the assembled but not-yet fired up build. This has been a fun one...

Cheers,

Paul

[Wombaticus]

I got this fired up, used the excellent HolmImpulse freeware to characterize the frequency response -- which look gratifyingly similar to the published curves for the original.. In the studio with balanced operation, the noise floor was somewhere below -90dB. So I think I'm declaring victory on this one and moving on! Wrap up is posted on the blog. Cheers, Paul

[kevinkr]

Please post some details here.

[Wombaticus]

Originally Posted by kevinkr

Please post some details here.

Yes, well, aside from the frequency response looking pretty good (and the unit therefore behaving like it's supposed to), I'm not sure there's a whole lot more to add.

To recap a few build-specific conclusions:

1. The Hammond 270CAX in conjunction with the 6X4 recto tube ends up delivering a B+ (with typical line voltages) about 25V below the schematic... which works out to about 7.5% less than nominal. So within 10%, and not a serious concern for me. Others might wish to take this into account by selecting a different PT, or by perhaps, altering the resistance in the PSU slightly. In my case, I used 47uF caps instead of the 40uF specified in the original, so I presumably could have chosen to compensate somewhat for the lower B+ by using smaller dropping resistors while maintaining the same PSU corner frequencies as the original. I didn't bother.

2. The strategy of running the 6.3V and the 5V windings in series to feed the DC regulator for the tube filaments appears to work fine, without any undue heating of the PT.

3. The combination of 2W metal film resistors, the new layout, and the addition of DC heaters seems to have paid off with a noise floor for the unit which appears to be somewhere below -90dB when run in balanced operation. It would almost certainly be quite good also with just AC heaters, but I confess to not having assessed this.

Cheers,

Paul