Passive OA-6 type 2

[Villee]

aHello!

I made my OA-6 type 2 working as passive, because my knowledge is not enough to fix the amp.
I swiched placement between mids and "woofs". With "woofer" I use a low-pass-filter and some extra damping behind the woofers to ape the "damping basket" used in OA-4/5.
So, I have a pair of speakers which has a bit better bass response (about the same as OA-5, down to 40 Hz) compared to my OA-4 and much clearer mids. My opinion is that, this is something you can live with (if you have a smaller room and tone controls in your amp!).
Next changing the capacitors from the midrange and perhaps replace the tweeters with better ones (a bit dull treble) and place them at Y-configuration.

Ville

[jansch]

Villee - sorry.... they are so nice when they are working properly.

Where are you living?

[Villee]

In Finland, near Helsingfors.

"Few" faults:
One of the bass-energizers has original transformer and the other one has toroidical 2x25 volt transformer and "kretskort" is pretty worn out.
One R19 has changed is value and V5,V6 are at short-circuit both of the energizers, C11 has a low resistance value (about 1kohm). Both of the energizers does NOT have R18 ,R10 and R22 has incorrect value.
Another faults: one of the speakers has one tweater dead (temporary solution is that I use now two tweets per speaker same way as in OA-12) and some crossover capacitor problems (weakened treble).

So pretty bad case

Ville

[Villee]

When using 2N5192/BD442 as a replacement V5 breaks when turning a on and then you have six to twenty volts output voltage depending on how badly V5 breaks and then 9710 get´s a wild ride... When using BD243/44, you´ll get a proper output voltage a moment but then V5 starts to smoke and fuses will burn. I suppose that R16 and R17 should get very hot when the amplifier is on, but in my case, these does not heat at all!
I can solace myself by thinking that these amplifiers are not too easy to repair (even that these are not too complex), so i did my best when I got a proper output voltage a Moment

Ville

[jansch]

Villee - WHERE are the speakers/energizers? In Sweden?

[Villee]

No...They are In Finland in it´s capital Helsinki/Helsingfors where I live.
SO shipping the energizers to Sweden to you for repair would perhaps be a complex and costly job when adding shipping-, compoment, and another costs.
On the another hand, it would be fine to send the energizers to someone who could fix them

Ville

[faulhund]

Hej to all, especially Villee and Jansch!

One has to consider that the "energizer" by today's standards is a very primitive design indeed with its six transistors. If yours are in a bad state, they are probably not worth while repairing. So, if you are not into restoring it to a "museal" state, why not replace them by integrated amplifiers and active filters? The popular "Gainclone" would probably be more than enough for a 9710. (I think of TDA2020 which was new, when I built my active boxes).

Unfortunately, I have found only an illegible JPG of the OA6-II amp., so I cannot do the reverse engineering for you, but I suppose you can straight-forwardly replace the 30 or 40 uF 6 dB/oct. HP for the mid and tweeters with an CR link before the mid-hi-amplifier. Is the LP for the woofer then also 1st order? (C11(?) of 0.1 uF // R8(?) of 22 kohm)? Then an RC link before the bass amplifier!

The main question mark for me is what to do with the "notch" filter in the feedback, I mean the 3 C and 3 R (which I cannot read) in parallel to R4, responsible for the peak somewhere in the vicinity of the BR tuning. Both its frequency and its peak gain are sensitively dependent on component tolerances, so this is another weak point of the design. (Considering that modern music is rich of bass, you might prefer to take back this very aggressive equalisation a bit, not to overload the poor woofer ...)
What do you say, Jansch, what should the transfer function of this peaking filter be? Something like the published curve for the "coalscuttle"? (It seems overly dramatic to me, since people object to a Q higher than 2 or so for a bass equalisation.)

Oh, yes, the negative output resistance! I almost forgot that, because it is really trivial to make with any modern amplifier with high open-loop gain, as I have described here: http://www.faktiskt.se/modules.php?name ... 784#776784. Provided that the amplifier is non-inverting, you only need a series power resistor (0.33 ohm?) between woofer and ground and a voltage divider to the non-inverting input. If you only watch out that the series resistor is small enough (see my suggested sizing in the link above, and the link in this link!), there is no risk of the stage going into self oscillation.

Good luck!

[Villee]

Here is a better quality schema from OA-6 type 2a (what I have). Or do you want a concrete picture from the energizer?
I presume your idea would be interesting, but some further advicing would be nice although I basically understand your idea.
For example, TDA2020 is well available and has a price about 3 euros.

[jansch]

I think you should choose an IC amp so you can use the existing 2x35V powersupply.

Why not use the same filter design including the positive current feedback?

[jansch]

Nice job from Grafpro.......

http://www.carlssonplanet.com/forum/vie ... oa6#p24867