helios preamp pcb's [boards now available]

[mus1k_freak]

I was just about to write back and say that sowter has some that are very similar, the original helios used lustraphone transformers, which are no more, maybe you could find a few vintage used ones if you really wanted to go original. Helios, ampex, quad 8, neve, etc all started fairly close to the same time, and a lot of the same people and designers worked on more then one brand, which is why some of them share similar topology. I think most input transformers would sound great with this pre, you could use cinemag, lundahl, ed Anderson, sowter, etc, each one would give you a unique flavor. I plan on running mine unbalanced out to save some money on the iron for a multichannel build, however its set up to drive a output transformer just fine, so if you would rather go that route it would be no problem to add  one to the output. its designed to run off of a +24 single rail, I included the power supply into the pcb board all you need is a 120 (or whatever your country has 220, 110 etc) to 24v, which is super cheap.

Which Q8 amps are these similar to? I looked at some schematics but the stuff I have (CA127, for example) looks like an opamp to me. 

Is there info on the input transformers for these (ratio/size/etc)?  Their -20 input is not a pad, but using the center tap of the input trafo secondary which I think is pretty cool. But does that imply a 1:10+10 input trafo? 


EDIT: found some info on sowter's site... their 7490 is supposed to be a replacement for the original and is 1:10 with a 1:1 tap...

 

[mitsos]

I'm getting excited about this project, this input trafo idea is pretty cool, and I never really noticed it before.  I have no experience with Helios myself, vintage or remakes, so I'm not going for original parts at all.  I'd prefer to try to wind an input transformer myself, or at least, have one made!   I'm curious as to how this 1:1+10 thing would be wound, and also how it will sound.  Maybe time for a new thread...

Have you (or anyone else here) used the lustraphones and maybe compared the 1:1 ratio to a more traditional resistive pad into 1:10 trafo?

If I were up north I'd volunteer to help, and could send you some output trafos to try... but they'll take months to get there from brasil... After all, it's carnaval and this year is the world cup...  :    But output trafos are cheap to add later too. 

 

[ruffrecords]

The Sowter 7490 is a nice little transformer. It is indeed a 1:10 ratio transformer and I used it in my early mixer and mic pre designs. It is unusual to see a 1:10 transformer in a semiconductor design but it does give you 20dB of nearly noise free gain.The 1:1 secondary tap is a bit of an anomaly.  It reduces the level going into the amp but not the level going through the transformer. I think Helios was a very cost conscious company and this is a simple way of doing a 20dB pad with a SPST switch rather than a DPDT one and three resistors. Note also, that their line input was unbalanced presumably also to save money. I talked to Brian Sowter about the 1:1 tap and he said from the performance point of view it was better to uses a regular 20dB pad before the primary. The Sowter 1948 is the 7490 without the 1:1 tap that Brian makes for me for the EZ tube project. If you want to use it just ask Brian for a quote. It should cost much the same as the 7490.

The only Helios I saw in the flesh was the one at Ramport Studios when we sold The Who the Neve that replaced it. It was unusal in that it had an unbalanced patch bay. This was in the form of an XY matrix not unlike a small scale peg board. Outputs were connected to columns and inputs were connected to rows. You could connect any input to any output just by inserting a peg in the right XY position.

Cheers

Ian

 

[mitsos]

I am still very curious to hear this preamp with both types of pad.

I wonder why Brian Sowter said it is better to use a resistive pad? Maybe the 1:10 winding presents a more proper impedance to the circuit?  I thought I read somewhere here that the lustraphone was chosen due to availability, not its quality (was it you that posted that? not sure).

Sowter's site says the case is 34mm in diameter, that's a lot bigger than the "little" API input trafo (25.4mm), if the core is actually larger too, it should handle more signal prior to distortion.  Now, assuming the sowter is a copy of the lustraphone (never seen a real Helios trafo), couldn't Helios have saved more money by going for a smaller core and a more conventional winding?

 

[mus1k_freak]

using a transformer winding as a pad is a cool idea, you dont really see that much. I ordered 20 boards for testing so ill have quite a few to give out to anyone interested in trying to build one, im going to use some of these as line amps to Ian's helios eq pcbs, figured it would be cool to have a helios make up gain board with a helios eq, should be pretty similar to the original channel strips. Ill be building one board on the cheap with a resistor pad and unbalanced output, and ill be building one with in and out transformers of the more expensive type and see how the sound compares between the two. after digging around some, this design is also very similar to a trident A range mic pre. it deals with gain differently, but the layout of the circuit seems very very close.

 

[mitsos]

that input trafo got me thinking.. but maybe I'm being naive and/or gearslutish.  The test for me would be with the Sowter 7490 set up with the trafo wired 1:1, and another wired 1:10 and running a -20dB resistive pad, so you can record the same source/take.  But that's 105 GBP (175USD) in trafos.. a bit much for a "test" I think.

Good to know you're getting a bunch of PCBs. I'll help out if I can, someone for sure will be in the US next month or so, if you have an extra pair of PCBs I'll send you an address and some funds later.  Or we can do a trade or something.

I had seen Ian's thread on the EQ but didn't know there was a PCB for it. I still haven't built my universal EQ, too much stuff on my plate... Should look at that again, maybe I can do the helios EQ on that?

anyway, keep us posted, cool project.

cheers!

 

[ruffrecords]

I am still very curious to hear this preamp with both types of pad.

I wonder why Brian Sowter said it is better to use a resistive pad?

It is quite simple. The flux in the transformer depends on the voltage across the primary winding. The bigger the voltage, the bigger the flux and the closer you get to core saturation and distortion, especially at low frequencies. So, if you use the 1:1 tap with a high level input signal you still have a high input voltage and risk additional distortion. If you use a pad before the transformer instead then the voltage is reduced on the primary, as is the flux and the potential for distortion.

Of course, you may want the added distortion, and it may even contribute to the overall Helios sound.

Cheers

Ian

 

[ruffrecords]

and ill be building one with in and out transformers of the more expensive type and see how the sound compares between the two.

The circuits you have posted so far are unfortunately not man enough to drive an output transformer. For that you really need to use the high level line amp schematic I posted.

There is one other quirk of the Helios preamps you should be aware of. Normally you would arrange the bias so that the output dc level is half the rail voltage in order to maximise the output swing. With a 24V rail this would give you around +20dBu max out. But the Helios biases  quite some way from half the rail volts. This reduces its output level and makes it clip asymmetrically. I seem to remember this limits its output to about +14dBu. The odd thing is, if you swap the two bias resistors across the first transistor, the bias sits dead on half the rail volts. This makes me wonder if the current bias arrangement is simply a typographical error made in the distant past.

Cheers

Ian

Cheers

Ian

 

[mitsos]

I am still very curious to hear this preamp with both types of pad.

I wonder why Brian Sowter said it is better to use a resistive pad?

It is quite simple. The flux in the transformer depends on the voltage across the primary winding. The bigger the voltage, the bigger the flux and the closer you get to core saturation and distortion, especially at low frequencies. So, if you use the 1:1 tap with a high level input signal you still have a high input voltage and risk additional distortion. If you use a pad before the transformer instead then the voltage is reduced on the primary, as is the flux and the potential for distortion.

Of course, you may want the added distortion, and it may even contribute to the overall Helios sound.

Cheers

Ian

Thanks Ian, sure, I understand that, what doesn't make sense to me that this strange transformer would be cheaper than a "normal" transformer + a switch+3 resistors. Also, knowing that it would distort more that with a conventional pad (they were recording rock and roll after all, the pad would have to be used on a lot of things).  In the end I guess we'll never really know why they did it, but it is fun to try to guess. I know I can't wait to test this thing.

As for the output trafo, do we know what output trafos might have been used? I've seen some pics online, it didn't look like there was one in the module itself.

cheers!

 

[mus1k_freak]

From what ive read and seen online the output transformers were in a separate part of the desk, that were more or less "remote" from these line/pre cards. the designed that I built this off of is the 2128 line amp that the helios desks used to make up gain from the aux sends, and the eqs et. I also included a separate transistor circuit to "drive" the output of the 2128 line amp, running these with a 1:10 input and a unbalanced out should give enough gain for most mic pre type applications. I debated adding a ne5332 balancing circuit to the output of this, but I don't think its gonna hurt to run unbalanced out. As for a output transformer this should be able to handle a 1:1 to balance an out but I don't know about much then that. Using it as a line amp or gain make up for eqs and things this should be great!

It's defineitly gonna take some playing with resistor values to get the bias right and things sounding as smooth as possible, ill start with whats on the drawings and take readings and tweak from there, ill try to get a BOM posted so if anyone wants to start colleting parts you can get a head start. There's a few different options for the last output transistor, I would suggest using a socket for that, as a few of the suggested types have different pin outs, so you can swap around and see what you like, im also gonna play with a few germanium transistors in the circuit as well, ive got a few oc44's I think laying around.

I am still very curious to hear this preamp with both types of pad.

I wonder why Brian Sowter said it is better to use a resistive pad?

It is quite simple. The flux in the transformer depends on the voltage across the primary winding. The bigger the voltage, the bigger the flux and the closer you get to core saturation and distortion, especially at low frequencies. So, if you use the 1:1 tap with a high level input signal you still have a high input voltage and risk additional distortion. If you use a pad before the transformer instead then the voltage is reduced on the primary, as is the flux and the potential for distortion.

Of course, you may want the added distortion, and it may even contribute to the overall Helios sound.

Cheers

Ian

Thanks Ian, sure, I understand that, what doesn't make sense to me that this strange transformer would be cheaper than a "normal" transformer + a switch+3 resistors. Also, knowing that it would distort more that with a conventional pad (they were recording rock and roll after all, the pad would have to be used on a lot of things).  In the end I guess we'll never really know why they did it, but it is fun to try to guess. I know I can't wait to test this thing.

As for the output trafo, do we know what output trafos might have been used? I've seen some pics online, it didn't look like there was one in the module itself.

cheers!

 

[mitsos]

do we know how the helios consoles did summing/mixing? Anyone have schematics?

 

[ruffrecords]

From what ive read and seen online the output transformers were in a separate part of the desk, that were more or less "remote" from these line/pre cards.

I think that is right. Helios consoles were pretty much unbalanced inside. The only balancing was on the inputs and the group & AUX outputs. As far as I know there were no direct outputs. This means their channel strips, unlike Neve channels strips, do not have a balanced outputs.
CHeers

Ian

 

[ruffrecords]

Thanks Ian, sure, I understand that, what doesn't make sense to me that this strange transformer would be cheaper than a "normal" transformer + a switch+3 resistors.

Winding a transformer with one additional tap on the primary makes no difference to the cost if the quantities are high.

I know an ex Helios guy. I will ask him if he knows why they used the tap method.

Cheers

Ian

 

[leadbreath]

count me in for any pcbs that are made...

 

[mus1k_freak]

I ordered 20 pcb's ill probably keep 5 for myself to play with so ill have 15 up for grabs from the first run, if they are well received and there's interested ill add any tweaks and changes and a second run. would love to get some feedback on these if people are willing to build them!

count me in for any pcbs that are made...

 

[hitchhiker]

I'd like a pair please!

 

[mjrippe]

Since you have an odd number, I'd be interested in building a single one.  I've got a few vintage and modern transformers to try out.

 

[mitsos]

Thanks Ian, sure, I understand that, what doesn't make sense to me that this strange transformer would be cheaper than a "normal" transformer + a switch+3 resistors.

Winding a transformer with one additional tap on the primary makes no difference to the cost if the quantities are high.

I know an ex Helios guy. I will ask him if he knows why they used the tap method.

Cheers

Ian

Let us know what the guy says.  Do you know how they did their mixing/summing on these boards?  passive like neves?

 

[ruffrecords]

quantities are high.

I know an ex Helios guy. I will ask him if he knows why they used the tap method.

Cheers

Ian

Let us know what the guy says.  Do you know how they did their mixing/summing on these boards?  passive like neves?

OK.

I do not know what kind of mixing they used but I suspect it was passive since there were no real audio op amps available at that time. THe NE5532 came out in the mid 70s.

Cheers

Ian

 

[mitsos]

Could they have used a discrete opamp like API?