Audio compressors and Plessey Vogad IC

[Wendymott]

Hi peeps.
I have made a SSB exciter with 9 +/- Mhz RF output into a Double Balanced mixer to convert to any amateur band.
However, I did intend to use the Plessey VOGAD audio processor, but when monitoring the SSB via my Eddystone 888 and using headphones to prevent feedback, I find the audio quality rather poor, more like "Donald duck". Inputting a sine wave into the modulator without the VOGAD, it sounds ok, thus I suspect the VOGAD.......... any comments please.
The VOGAD chip is the early one SL622.... I wonder if the later one was improved, which I dont have

[G8HQP Dave]

Maybe try adjusting the time constants to suit your voice?

[Radio Wrangler]

The gain control element was a bit crude in the first ones and quality was only just tolerable for military manpack radios.

David

[Wendymott]

Hi peeps... Thanks for your comments..... I will look elsewhere.. Looks like the military were not too bothered about quality...

[Restoration73]

The later SL6270 works ok. There are stocks of these around.

[VT FUSE]

I know that these VOGAD ic's found extensive and long service in the Clansman series of Battlefield comm's.

It a fact that a fair number of 'reject-out of spec' chips in the early round can 10 lead metal packages were offered for experimental use by a well known and respected trader at radio rallies etc.

I find that I have two SL6270C 8pin dil packaged devices,one in RS sealed bag and another in a barcode traceability type poly wallet.

If you should like either one,f.o.c. please pm me

[Wendymott]

Thanks all.... Mike has kindly offered to send me a SL6270C...

[boxdoctor]

Hi Wendy, If you are still having problems after trying the other Plessey chips, may I suggest you try a BA3308. I have used these for several different projects over the years with no snags at all. S.I.L encapsulation, so lends itself well to veroboarding. Only snag is they are stereo, but you can use only one channel, as the two halves are independant of each other (apart from the common ALC line). About £1-50 from Cricklewood. Tony.

[G6Tanuki]

How are you connecting the mike?

In the likes of the PRC320 the VOGAD chip is used as a balanced-input device.

If you're driving it unbalanced with one side of the mic earthed, you need to decouple the unused input to earth with a suitable capacitor to get proper audio performance.

[Radio Wrangler]

Balanced operation is also used to get quite a bit of reduction of even order distortion terms. I had quite a bit of trouble with the discrete VOGAD arrangement in a Redifon GK203.

"The Hitch-Hiker's Guide To The Galaxy has this to say on the subject of VOGADs:"......

RF speech compressors are a lot better.

David

For those with a bit of microcontroller experience see this http://www.microchip.com/SWLibraryWe...codingDecoding

[m0cemdave]

I have often wondered about the connections to these IC's.

The Plessey application manuals say the inputs to an SL622 or SL1626 should be capacitively coupled and must not be connected to a source of DC. Balanced input has an impedance of (a rather low) 300 ohms and will handle up to 375mV. Unbalanced impedance is only 180 ohms and will handle up to 50mV; in this case the unused input must be decoupled to earth.

The same applies to the later version, SL6270. This chip was used in various amateur radio construction projects and I have often wondered about the connections. Some diagrams show the unused input as open circuit, others show it decoupled with 2.2 uF and a resistor of either 22k or 47k. They all seem to work, but I wonder what is optimum?

Most general purpose dynamic microphones have an impedance of around 150 - 200 ohms and should see a load of at least 2k for optimum performance. Loading one of these with 180 ohms will result in a loss of 5 or 6 dB of level.

p.s.
If using a resistor between pins 7 and 8 to set the basic gain, the coupling capacitor value needs to be increased from the original 4n7 uF. The Howes CM2 mic preamp kit failed to do this and it wouldn't work with the supplied component values!

[m0cemdave]

Originally Posted by m0cemdave

the original 4n7 uF.

(too late to edit) ... sorry, 4.7 nF

[Wendymott]

Hi peeps.... I lashed up the SL6270 yesterday am with pin 5 grounded via a 22K as per one application diagram.... fed single ended from a 600 ohm mike via a 47uF cap...to get enough voltage to modulate the SSB mixer I used a TL072 /2 with a gain of X10 and a bit of capacity in the feedback across the 100k (1nF) to roll off the HF. This is NOT by any means the final version, I need to look at a better filter to define the frequency response.. I think it should be 600 hz - 3 Khz.. Things are looking better....
Looking at the Alinco DR150 FM tranceiver Mike input circuit... it has audio filtering to limit the frequency response so it is a starting point

[Wendymott]

Here is the completed version... used the VOGAD chip and 2 x TL072's good roll off below 400 hz and above 3 Khz.
Band filtering and PA next