Hammond 229A56 transformer

[ruffrecords]

I bought one of these transformers recently for a couple of reasons. First to use as a phantom supply in a project but second to examine it physically because it is a low profile transformer that should fit nicely into my new 35mm wide tube mixer modules.

http://hammondmfg.com/229.htm

The one I have is a 6VA type and it is only 22mm high. Since it has dual primaries and secondaries it is possible to wire it as a 115:56 which is very close to 2:1 which is the ratio I use on the output stages of my tube mixer designs. With it wired this way I measured the primary inductance 1t 100Hz as 10H and the secondary inductance at just over 5H. This is puzzling as it implies a 1.4:1 ratio not the expected 2:1.

Next I tested it using my Lindos audio test set. Initial test was with the secondary loaded with 600 ohms. The results were disappointing. 1KHz level was 11dB down. Frequency response was fine from 20Hz to 100Hz but it was 3dB down at 600Hz and about 33dB down at 10KHz.

So I tried it with no load. 1KHz was now down 3dB (which is what you would expect from1.4:1 ratio). Frequency response was essentially flat except for a big resonant peak at 20KHz.

Lastly I rewired the primary for 230V and measured that actual secondary volts, unloaded and loaded with 550 ohms. Unloaded the secondary voltage was 80V rms. The mains measured 238V which gives a nominal 3:1 ratio. When loaded with 550 ohms the output dropped to 58V rms which gives it a regulation of about 30% - probably not surprising given its size and construction.

A couple of things puzzle me. It has two separate bobbins, one on each arm, so for best safety insulation you would expect both primary winding to be on one bobbin and the two secondaries on the other. But they are not; each bobbin appears to hold a primary and a secondary based on the fact that the primaries come form both windings (as do the secondaries). Hving siad that, now that I have removed the first layer of tape from the bobbins it is clear that each bobbins has two separate sections so the primary and secondary windings are physically separate.

The other thing that puzzles me is the loaded frequency response - it drops off a cliff after 600Hz. I know it is not intended to pass anything other than 50/60Hz but its unloaded response is not so bad. Is this due to it being a kind of C  or U/I core?

Cheers

Ian

 

[volker]

A couple of things puzzle me. It has two separate bobbins, one on each arm, so for best safety insulation you would expect both primary winding to be on one bobbin and the two secondaries on the other. But they are not; each bobbin appears to hold a primary and a secondary based on the fact that the primaries come form both windings (as do the secondaries). Hving siad that, now that I have removed the first layer of tape from the bobbins it is clear that each bobbins has two separate sections so the primary and secondary windings are physically separate.

You want both legs to be balanced and identical. Hence both the primary and the secondary are split in half and wound on both bobbins. And as you say the bobbins are each divided for isolation/safety reasons.

The other thing that puzzles me is the loaded frequency response - it drops off a cliff after 600Hz. I know it is not intended to pass anything other than 50/60Hz but its unloaded response is not so bad. Is this due to it being a kind of C  or U/I core?

A consequence of having the primary and secondary winding next to each other (talking about the fact that the bobbin is split, not that there are two bobbins) instead of on top of one another is high leakage inductance, because the coupling is reduced. When you look at the equivalent circuit of a transformer you will see that the leakage inductance basically acts like an additional L-filter in series (or LC in combination with the winding capacitance), and so reduces the top-end frequency response.

 

[ruffrecords]

You want both legs to be balanced and identical. Hence both the primary and the secondary are split in half and wound on both bobbins. And as you say the bobbins are each divided for isolation/safety reasons.

It is not clear to me why I would want both legs to be balanced and identical

A consequence of having the primary and secondary winding next to each other (talking about the fact that the bobbin is split, not that there are two bobbins) instead of on top of one another is high leakage inductance, because the coupling is reduced. When you look at the equivalent circuit of a transformer you will see that the leakage inductance basically acts like an additional L-filter in series (or LC in combination with the winding capacitance), and so reduces the top-end frequency response.

Yes, of course.; leakage inductance is much increased because of low coupling. Leakage inductance is effectively in series with the output to which explains the poor loaded response.

So if I wanted to use this form of transformer for audio I should use a single bobbin on each side and bifilar wind primary and secondary on each. I have seen a transformer of this style used in a 35mm Neve module so I guess it must be possible.

Thanks for your input.

Cheers

Ian

 

[Tubetec]

I wonder what would happen to the frequency response if you could re arrange the coils , the way it is ,its two secondaries at one end two primaries at the other ,

    PP
    SS

how about the case below

    PS
    SP

I've seen these low profile power transformers alright , but never though about repurposing or rewinding for audio . I think I know where I might be able to  get a few for experimental purposes .

 

[volker]

It is not clear to me why I would want bath legs to be balanced and identical

In order to cancel leakage flux and also hum. Not sure if I remember right, I think it was from a Crowhurst article, but there is also some distortion mechanism related to this.

So if I wanted to use this form of transformer for audio I should use a single bobbin on each side and bifilar wind primary and secondary on each. I have seen a transformer of this style used in a 35mm Neve module so I guess it must be possible.

Not necessarily bifilar, but interleaved windings. It depends on the particular application of course. There are plenty of examples from CJ's dissections that use UI laminations, just naming a few: UTC A-20, UTC A-26, Freed 32035, Newcomb TR-91, McCurdy MRI 2001.

 

[Tubetec]

So are we saying that each of the four bobbins  would contain identical interleaved/spaced primary and secondary sections ,which could then be connected series/parralel as required ? 

 

[volker]

It's possible, but rarely done I think. I've only seen two examples of audio transformers with split bobbin, one is the Telefunken output transformer for 2x EL84 in the attachment (CJ might know more). Wound on a EI 78 core, 1/5 is secondary, 2/4 primary, 3 is a feedback winding. The other was a East German broadcast preamp output transformer I unwound because it was burnt to death with mains voltage. On both transformers the windings are parallelled and cross connected to ensure proper coupling. The advantage of course of a split bobbin is the inherent symmetry. Which goes for EI, on the UI the same symmetry is achieved by using two bobbins.

 

[ruffrecords]

In order to cancel leakage flux and also hum. Not sure if I remember right, I think it was from a Crowhurst article, but there is also some distortion mechanism related to this.

Did not know you could cancel leakage flux. I will look further into that.

Not necessarily bifilar, but interleaved windings. It depends on the particular application of course. There are plenty of examples from CJ's dissections that use UI laminations, just naming a few: UTC A-20, UTC A-26, Freed 32035, Newcomb TR-91, McCurdy MRI 2001.

Thanks for the suggestions. I will check them out.

Cheers

Ian

 

[rackmonkey]

Quote
Not necessarily bifilar, but interleaved windings. It depends on the particular application of course. There are plenty of examples from CJ's dissections that use UI laminations, just naming a few: UTC A-20, UTC A-26, Freed 32035, Newcomb TR-91, McCurdy MRI 2001.

Thanks for the suggestions. I will check them out.

Late to this, but it’s an interesting topic.

Ian, if you look in some of the old Thordarson catalogs (1942 catalog for certain), there’s a drawing of the Tru Fidelity series construction, which were all done using U/I cores with dual bobbins. They are advertised as humbucking, which makes sense. Though not diagrammed, the Chicago catalogs from the 50s describe their full spectrum models as being constructed this way as well.

I’ve noticed that many of the Jurassic era classics for broadcast/recording use (that I’ve seen evicerated or diagrammed, anyway) have this construction. Even the ones with multiple mumetal shields. It makes me wonder why this construction type seems to have faded from use in modern transformers for similar applications. Was the noise environment just that much worse that dual bobbin/U-I humbucking types were deemed necessary? Or was the cost/benefit ratio just not compelling enough?

i’m talking mainly about output transformers not using this design pattern today. I believe it’s still at least somewhat common for input transformers, as I’ve dissected some Altec 1689 mixer transformers from the 70s, a couple of Sescoms from the 80s or 90s, and a modern Cinemag (the CMMI-7C, I think) that had open windings, and they all had this construction, which would make sense, since inputs would seem to benefit most from the additional noise cancellation the dual opposed windings provide.

Wondering if our newest guru Bill W has insights here. But our other experts may as well.

Edit: I realize the dual bobbin U/I core construction isn’t the only way to achieve the noise cancellation I refer to above. The main question here is why we don’t see many transformers built like this any more when it used to be very common.

 

[volker]

Lundahl uses a lot of amorphous cut cores, which function on the same principle of "two legs". Other than that, you don't really know what's going on in many of the potted transformers.

 

[ruffrecords]

The reason I am interested in UI cores is they look like a good route to low profile transformers. Back in the 70s, Neve introduced their 35mm modules. Initially these had to use external output transformers simply because they were to big to fit inside the new modules. Not much later they introduced versions with integral output transformers and these used UI cores. One significant advantage of a UI core over an EI core is the UI core has two winding limbs instead of one. This means that in the same physical space you can have twice as many turns. This is important in output transformers where higher voltages are required because he voltage the core can sustain before saturating is proportional to the number of turns.

It seems to me that these characteristics are ideal for designing low profile output transformers.

Cheers

Ian