Altec 458A Build

[DaveP]

After some conversations with EMMR I decided to have a go at recreating an Altec 458A
I liked the simplicity and lack of feedback, only problem was there was no chance of me getting hold of the original transformers.  First job was to buy a copy of the AES paper written by the designer of the circuit.

After doing this I was able to estimate that the input TX was about 1:15 and the output 6.45:1.

I decided to use a Sowter 9530 15k:600 (ignoring the feedback windings) and 2x OEP187X TX's

I managed to get it into a 1U case using toroidal power transformers.

You can see the two input TX's

Finished job

The gain varies depending on what you connect it to because there is no feedback, just like the original.  So it can be 34 to 40dB, very good for a dynamic mic but too much for a condensor without a pad.
When it was first powered up there was an oscillation at 60kHz but this was cured by the two Zobels across the input secondaries, also gave a half decent square wave afterwards as well.  The two cross connected caps were not needed with the Sowter TX.  Noise was about -83dB below 0dB, which is good for a non feedback amp and is not heard in normal use.
thanks to EMMR for help and advice
best
DaveP

 

[MagnetoSound]

Another fine job there, Dave!

I love pop rivets, don't you mate?  ;D

 

[emrr]

Thanks for the report, interesting to see you didn't need C1/C2 after all. 

150:150K input mic to grids was a real revelation to me; that is somewhat beyond what is considered possible for good fidelity, yet Peerless did it.  I suspect from personal observation that this manages to work so well due to PP operation and balance, and would fall down badly if attempted in a single ended circuit.  25K:600 output was revealed by PRR in a post way back; now the picture is more complete. 

 

[chrispbass]

Great work Dave, have been impressed by your other builds too, very creative and well thought out.

I was wondering where you get the chrome rack handles from as i'd like to get some too for some of my builds?

best

Chris

 

[DaveP]

Thanks Chris,

the handles are from Farnell:
http://uk.farnell.com/mentor/270-8/handle-round-30mm/dp/3058505?Ntt=305-8505

They call them steel but they are chrome plated brass

best
DaveP

 

[chrispbass]

Superstar 

 

[emrr]

Now, to tell us what you think about the sound?

 

[DaveP]

I was hoping no-one would ask me that Doug as I haven't done a proper back to back test yet!

I normally record vocals with a Rode NT1-A but it was too potent for that, so I used an SM57 instead and it sounded very good to my ears, then I tried it with an AKG 770 which sounded much better than the SM57.

I've been reading a transformer book by a Dutch Guy who came up with an interesting bit of info.  If you have a really good transformer that can reproduce a perfect 40Hz it turns out that it does not sound so punchy as a poor Transformer with 2nd Harmonic 80Hz and 3rd harmonic 120Hz mixed in.  In fact if you filter out the 40Hz the ear/brain reproduces the 40Hz from the 80 and 120Hz!  It sounds weird but the brain lets us see white from a combination of RGB on a TV screen so it doesn't surprise me if it can do similar tricks with sound.  I wondered if this was the key to why some vintage transformers sound good?
best
DaveP

 

[emrr]

Yes indeed, the mind fills in the missing bits.  Speakers are the strongest example, since most can barely do any real lows at all. 

Listening takes time, a gentle prod.   

 

[DaveP]

I take the hint, but its past my bedtime,
I'll answer any more posts tomorrow
zzzzzzzzzzzzzzzz
over and out
DaveP

 

[MikoKensington]

Very nice work.  I've always wanted to hear a 458 myself. 

 

[lassoharp]

Great work Dave!

 

[zebra50]

Nice build - I like that case with the two compartments.

Interesting use of the transformers too - what was the thinking behind using the two input trannies rather than finding a single one? Was it just forced on you through availability issues, or are there some other advantages or drawbacks?

Stewart

 

[DaveP]

Interesting use of the transformers too - what was the thinking behind using the two input trannies rather than finding a single one? Was it just forced on you through availability issues, or are there some other advantages or drawbacks?

Hi Stewart,

I simply couldn't afford to have Brian Sowter or anyone else make a special for me as the ratio is so high.    There is also the problem of the secondary centre tap, amazing how few manufacturers put them in.  I've used the OEP X187's for about 40 years, they are cheap and easily obtainable from RS, you can get up to 25kHz from them, and although there's not much below 40Hz that doesn't really bother me.  By using two like that you get 1:18CT.  There must be mutual coupling through the paralleled primary which results in the 60kHz oscillation but as I said, it completely goes using the zobels on the secondary.
best
DaveP

 

[emrr]

No one has marketed a "hi quality" 150:150K in many decades. 

 

[Kingston]

Very nice build. Perfect p2p layout work, once again.

No one has marketed a "hi quality" 150:150K in many decades.

Or is the question really: does anyone even possess the knowledge to wind them anymore?

 

[emrr]

Sure, the knowledge is there, enough buyers at the necessary price aren't. 

 

[plumsolly]

Very nice looking build - you should be proud of that one. Best, Ben

 

[DaveP]

Thanks for all your kind words Guys

A little encouragement works wonders

best
DaveP

 

[lassoharp]

Thanks for all your kind words Guys

A little encouragement works wonders

best
DaveP

Alright then,  I'm going to really egg you on to go after a Collins 26C.  You'll get a whole heap of cheering for that!!!    ;D