904a query

System 55 • System 35 • Model 15
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Rarecomponentadsr
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904a query

Post by Rarecomponentadsr » Thu Dec 03, 2020 12:05 pm

I’ve played a few 904a filters both genuine and cloned and also the new Behringer 55 series.
I’ve noticed that most have a good high frequency response when adjusting frequency control ..at fully clockwise (no regeneration) a saw wave starts to “hiss”
at the extreme top end. The Behringer 904a’s have this characteristic.
However, one or two 904a’s I’ve played including one genuine Moog 904a dont have this characteristic. They seemed a little on the dull unresponsive side.

I’m assuming that it’s the ladder section of the circuit that determines the high end respons ? Or does it lay somewhere else in the circuit such as regeneration section ? Although tuning the 904a makes an effect on overall sound it doesn’t seem to effect what I’ve described above .
I’m not using any regeneration on these tests.

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analogmonster
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Re: 904a query

Post by analogmonster » Fri Dec 04, 2020 3:19 am

Do you mean "hiss" in the sense of a kind of noise? If so, this might be a kind of overdrive effect. Perhaps it depends on the saw amplitude for different 904a implementations.
But is this an effect you want? My clone does not start to "hiss" in the highest frequencies, but adds a soft "purr" to the upper spectrum which I like most. Apart from that, the output is a clean saw to the point of the frequencies where I don't hear anything any more and the dogs in the neighbourhood start barking...

Rarecomponentadsr
Posts: 115
Joined: Mon Aug 24, 2015 5:08 pm
Location: Derbyshire England

Re: 904a query

Post by Rarecomponentadsr » Fri Dec 04, 2020 11:50 am

Thanks for reply..

Yes, it’s hard to describe. I’ll try.
Similar to the Treble control of a hi fi where on some, the full clockwise
gives a very biting and harsh tone with a hint of ‘buzziness’.

I follow your description of a saw tooth being normal going thru a 904a as you turn the Frequency to full CW. I’ve played a few 904’s that are similar
but as described some, like the new Behringer 55 904a , adds a sort of a hiss about 2-4pm on the dial or should I say the saw wave is there but there’s an added shhhhhhhh a sort of white noise effect where the frequency goes way up. It’s not an added noise, merely the frequency biting hard.

It’s a small detail but I’m very curious if original 70’s 904s had this same attribute. I’m also wondering if the sound is inherent with SMT components
and ultra modern lay outs SM ICs etc enhancing response.
Having said that, a 60’s 904a I once played also had the effect !
On the other hand one 904a I have (not original) sounds rounded and dull the full frequency travel.

Lots written about self oscillation, some do some dont etc but no mention as Ive seen of the behaviour of the Frequency control.

Apologies for trying to describe a sound source in words ! I’ve done these rough tests with same identical saw wave.

Maybe someone with a Behringer 904a could concur ?

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