Scully 280 preamp , transformer noise issues.

[maxwall]

I'm looking for a compact low stray field power transformer with at least 500 MA output or so.

toroid or standard.

every transformer I have tried seems to make noise. I'm trying to power a unit with a internal
slide in cassette power supply of my own creation, but I get varying amounts of hum in my audio path.

I have tried a open frame transformer as well as enclosed steel framed. Obviously the open frame transformer was the noisiest of the two.  I have not tried a toroid yet.

I'm trying to design this power supply so it is not a separate box. I dislike having a separate box sitting on the floor with a power chord.  I know its quieter , but too much inconvenience having two boxes to lug around.

nobody likes a preamp with a anchor.

 

[keefaz]

Maybe you could try shielding the hum sensitive parts of the circuit rather than trying to reduce noise on the transformer side ?
What sort of audio circuit is it?

 

[maxwall]

This is a Scully 280 Head amp for a 2 track reel to reel . Its a mono unit which makes a really impressive discrete UTC transformer coupled preamp.

I have  designed a slide in cassette power supply in a compartment that housed a slide in oscillator card with edge connector- no other cards in the compartment , only one. The oscillator card is not used when coverting to preamp. I removed the card and this gave what seemed to be the perfect isolated steel surrounded compartment that looked ideal. However , at the end of the compartment where my supply transformer resides, it is picking up the stray transformer fields and Its pretty obvious that keeping the transformer as far as possible to the end of the compartment is the key. However space is my limiting factor. I'm trying to position the transformer at different angles,to get some reduction in noise or hum.

I just tried a Amveco Toriod transformer and after three attempts with three different transformers of
different designs and manufacturers, I still get a low level noise that finds its way into the audio path. If I draw the cassette out of its compartment, the noise reduces. When I get the transformer about 10 or so inches away from the location I designed it for, the preamp is dead quiet.

I was hoping someone knew of a trick to restrict or eliminate the noise by using a special power transformer with a really low stray field so the hum will be significantly reduced. But this sounds expensive. It would be great to find a easy way out of this problem without having to resort to exotic shielding materials, a road I don't want to go down. There might be a simpler solution. I just hav'nt hit on it yet.

I'm looking at moving PCB PSU and transformer around the inside of the cassette to get a happy medium of idle noise.but its very time consuming and tedious work. I have already done this several times and all the work is leading me to wasted time since I get the same old results over and over - transformer stray field noise.

I have even cut pieces of 16 ga steel sheetmetal to block the field in the back of the compartment , but to no success.


maybe I hit a stalemate with this cassette power supply idea, not sure. I'm not willing to give up so easily.

the power supply cassette measures 6"L X 3.75"W X 3" H

any suggestions ?

 

[keefaz]

Try send a PM to Bo Hansén (a member here) or email him through his site: http://www.hansenaudio.se/#contact. I think he might know
and from experience, he does reply quickly to email
Or maybe edit your title like "Noise issue with Scully 280 Head amp" or something like that, so people who know this circuit could help

 

[maxwall]

We'll after more attempts, I finally stuffed that cassette the correct way and now I have very quiet
results. I wound up using a low profile Amveco Toroid and re-positioned it at a different angle. The pcb board had to be mounted on its edge with a cutomized bracket. Sounds much better than before and no
detectable hum from any stray fields of the power transformer.

It took too long to find just the right position for the transformer and PCB

The transformer wound up being mounted on the end of the cassette opposite that which gave the most
noise - I knew there was no other solution. The layout inside the cassette had to be completely re-done. About 5 times in all with the different transformers I used. The Toroid was angled totally opposite of the way I had it initially.

Most of you reading this have probably experienced the same issues , all following nearly the same rules. However, each project is slightly different with its own independant challenges, space contraints and many other elusive problems that must be overcome by trial and error to get the look ,sound and feel just right.

best of all , I don't like hacking or defacing original gear like this when doing the conversion. This way the headamp can be re-converted back to use with the reel to reel it came from with minimal effort and the look and feel are'nt ruined or permanently scared.

I'll post some pics soon so you can see the final outcome.

 

[Silvas]

BTW, i love my Scullys !

DIY´ed power supply for my 4 units...for -24 and +48v

I removed the big red pilot and put a rotary switch instead...so i can give phantom to either mic or line transformers, actually i´m using the line input with hot condensers a lot more than the quite sensitive mic input...really nice preamp !

I wanted to do an internal PSU just like you...then i realized that for 4 units an external PSU rules.

 

[maxwall]

One note on these Scully 280's is that everyone complains about the Mic input being too hot and needing some padding.

If you look closely at the Mic input transformer ( UTC A39 ) its primary strap is set for 150 ohms impedance, the second choice would be to change that to 600 ohms and this will take care of the padding needed to tame the hot input. impedance choices for the primary are 150 and 600.

 

[maxwall]

Here's the photo's

rear view w/ cassette power supply

Front view

 

[Silvas]

One note on these Scully 280's is that everyone complains about the Mic input being too hot and needing some padding.

If you look closely at the Mic input transformer ( UTC A39 ) its primary strap is set for 150 ohms impedance, the second choice would be to change that to 600 ohms and this will take care of the padding needed to tame the hot input. impedance choices for the primary are 150 and 600.

Cool !

I have to try that !

You think it will affect the sound too much wiring the primaries in series ?

 

[maxwall]

Affect the sound quality you mean  ? not sure, something to try out. It can even be combined into
a rotary impedance switch for the input.

Also,  I noticed that the current draw for this preamp was about 175 Milliamps measured.

If you look at a schematic of this preamp you will also find it has a Bias filter after the
600:600 Freed output transformer. I'm wondering if this really needs to be in there. Is
a rather elaborate filter of sorts. I'm not used to seeing such a filter on preamps like these
and other's. maybe its more than is really needed for just a preamp.

Here is the short hand version of the bias filter.

 

[Silvas]

Affect the sound quality you mean  ? not sure, something to try out. It can even be combined into
a rotary impedance switch for the input.

Also,  I noticed that the current draw for this preamp was about 175 Milliamps measured.

If you look at a schematic of this preamp you will also find it has a Bias filter after the
600:600 Freed output transformer. I'm wondering if this really needs to be in there. Is
a rather elaborate filter of sorts. I'm not used to seeing such a filter on preamps like these
and other's. maybe its more than is really needed for just a preamp.

Here is the short hand version of the bias filter.

My current draw is about 70mA per unit, without the bias card (which have some nice caps i´m using for bypass on other things) but i´m using leds instead lamps on the meter...

That filter...we should ask Bo. I´m sure is there for a reason, but why?


BTW your internal PSU looks cool

 

[maxwall]

Yeah the Oscillator card had two 10000pf /500V styrene caps , the largest I have ever seen.

the 175ma draw I measured was without the Oscillator card and regular incandesant light bulbs.

seems kinda high on mine. maybe the Leds drop it way down.

 

[Silvas]

Yup, it is the lamp that sucks alot of current. I measured before and after. BTW, i'm heating the iron in this right moment to change the mic input trafo ratio on 2 of my 4 units, from 150 to 600 ohms. And i have to solve some oscillation issue on one of them Seems related to the playback card.

Best regards

 

[gevermil]

curious what you guys find out . I was thinking of  the output section of this amp and how it could be toyed with .

 

[Silvas]

Yup, it is the lamp that sucks alot of current. I measured before and after. BTW, i'm heating the iron in this right moment to change the mic input trafo ratio on 2 of my 4 units, from 150 to 600 ohms. And i have to solve some oscillation issue on one of them Seems related to the playback card.

Best regards

Done

I changed the input trafo from 150 to 600 ohms on 2 of my 4 units.

As expected, sensivity was reduced. Now i have to try them on real life situations.

And i found a bad 300uf cap before the freed output trafo...changed it and solved some oscillation issues on one of the units.

 

[emrr]

One note on these Scully 280's is that everyone complains about the Mic input being too hot and needing some padding.

If you look closely at the Mic input transformer ( UTC A39 ) its primary strap is set for 150 ohms impedance, the second choice would be to change that to 600 ohms and this will take care of the padding needed to tame the hot input. impedance choices for the primary are 150 and 600.

150 to 600 swap is virtually never more than about 4 dB total sensitivity change.    I have seldom found a situation with vintage gear where I liked the sound of a 600 ohm tap over a 150, and padding situations seem to always need more than that will induce.  But, I can't speak directly to these units. 

 

[Silvas]

One note on these Scully 280's is that everyone complains about the Mic input being too hot and needing some padding.

If you look closely at the Mic input transformer ( UTC A39 ) its primary strap is set for 150 ohms impedance, the second choice would be to change that to 600 ohms and this will take care of the padding needed to tame the hot input. impedance choices for the primary are 150 and 600.

150 to 600 swap is virtually never more than about 4 dB total sensitivity change.    I have seldom found a situation with vintage gear where I liked the sound of a 600 ohm tap over a 150, and padding situations seem to always need more than that will induce.   But, I can't speak directly to these units. 

Yep, emrr, you are right.

Not a big change (level-wise). That´s where my -20 pads and phantom-powered line input trafo come in handy. The only thing i´ve been able to record with a 280 without a pad using the mic input without overload is acoustic guitar (with km84 or 451)  or room mics with RCA 44 mics.

Sound-wise i still don´t know. I usually like primaries in series but i have to listen yet.  And compare my two 150 ohm units with the 600 ohm units.

 

[maxwall]

Silvas,

I would be interested in your opinion on the primary transformer impedance change.

Also, does your VU meter have little movement when signal is passsing thru the unit or is it
quite active with a lot of movement ?

What are your knob settings ?

I have no other units at hand to compare to, so your help would be great.

Which defective cap did you replace and what were the symptoms if you could describe them.

I connected up a condenser mic with mine and used the mic padding to get the sensitivity way down.
seemed to work good without having to add padding to the 280 internal circuit. Sm58 is a different story.

I still think it would be nice to put the primary transformer on a 150/600 rotary impedance switch if the two
impedances make any worthy changes in the way the input sounds , that is. maybe its pointless, but worth
experimentation.


Gevermil,

What changes to the output were on your mind ?

emrr,

you point is well noted and probably universally correct with this type of equipment.
there is lots of room for experimentation with these 280's since there is more circuitry in them
than is probably required due to their intended purpose as a head amp for a reel deck rather than a
simplified preamp.

 

[Silvas]

Hi

I´ll report on sound as soon as i have a chance...i´ll be exclusively doing mixing the next 2 weeks, so not much chance of using the scullys on a session.

My VU started to oscillate after a loud peak with gain on 6-7 and the pre self-oscillated with the gain at max. Not audible but i saw the cones of my monitors moving....it was definitely related to the bad cap It was one of the 300u/3v before the ouput trafo. I replaced it with whatever i had at hand (470uf) but i´m ordering the correct caps. I recapped all my 4 units, following the schemos and signal flow (about half of the caps on each card) but i forgot to order the 300uf/3v !

The rotary idea is cool...sadly, the only spot for a rotary on my units, now is used by a rotary for :  (phantom off - phantom mic - phantom line).

 

[maxwall]

Silvas,

Ok , look forward to hearing back on that