Audio Transformer - UK Vintage Radio Repair and Restoration Discussion Forum

ms660 · 6th Feb 2016, 10:16 am

Trying to design a circuit, need to step up the output from a mixer (RF + LO in...AF out) The mixer output is designed to see a 50 ohm load, I need to get the AF output from the mixer to the grid of an AF amp, I'm thinking of a step up transformer, need max volts at the grid, frequency response not critical as the AF will be filtered down to 5Khz or less somewhere in the chain... Microphone transformer ? Cheaper alternative? Don't really want to go down the grounded grid route.

Any thoughts folks.

Lawrence.

Radio Wrangler · 6th Feb 2016, 10:26 am

An AF transformer connected straight onto the output of a mixer will have a dodgy impedance with all sorts of stray resonances at the RF and LO frequency.

Most mixers give their best performance into a defined and resistive load.

Use a diplexing filter after the mixer so that the input frequencies of the mixer and its LO range are connected to a load resistor to keep it happy, and lower frequencies go to the transformer.

If you're building a direct-conversion receiver, the signal out of the mixer will be rather low, and low noise performance under the condition of the low impedance presented by the mixer is needed. This is an application where the transistor has a big advantage as well as saving an awkward to find transformer.

David

ms660 · 6th Feb 2016, 11:24 am

David, yes it's to do with a direct conversion receiver I want to build that I've rambled on about in other sections of the forum, don't ask me why but I want to do a hybrid job, no transistors in the RF/Mixer or AF, just valves on 12 volts HT, the hybrid bit comes in with the LO, I've order up a DDS VFO job.

What I was thinking about was grafting a step up transformer into the output of a diplexer/filter of the type shown in Fig 4 here:

https://www.arrl.org/files/file/Tech...df/9208019.pdf

Can it be done or will it throw up other problems?

Lawrence.

G6Tanuki · 6th Feb 2016, 11:52 am

Quote:

Originally Posted by G6Tanuki

Make the first audio stage grounded-grid, with the signal from your balanced mixer going to the cathode. Depending on the valve you use, the input-impedance at the cathode could well turn out to be pretty close to 50 Ohms.

Similar things were done in the early days of amateur VHF: I've seen several designs where the centre of an incoming 50/75-Ohm coax has been coupled to the cathode of the first RF valve via a simple series-tuned circuit. The anode circuit is of course high-impedance: essentialy the grounded-grid stage is being used as an impedance-matcher more than a pure gain stage.

You can do it (and its cathode-follower) in a single envelope: https://www.gearslutz.com/board/atta...e-follower.png

Radio Wrangler · 6th Feb 2016, 1:12 pm

Hi Lawrence,

Rick Campbell doesn't often miss a trick, but he missed on there. He's made the diplexer pass audio and reject everything above audio. Which means large inductor and capacitor values which means components which will be bad at RF. It would be better (and easier) to pitch the diplexer higher in frequency to split the audio off somewhat below the lowest RF frequency to be used. The purpose is to give the mixer a nice termination at those frequencies which will come out of it, and not to reflect them back in (which causes degraded mixer performance).

David

ms660 · 6th Feb 2016, 1:45 pm

Would this be better? (Fig. 10):

http://www.robkalmeijer.nl/techniek/...991/04/page39/

Lawrence.

Radio Wrangler · 6th Feb 2016, 2:42 pm

Lots!

That's a very good article.

David

ms660 · 9th Feb 2016, 10:54 am

When I get some bits together I will try a resistive 50 ohm termination on the mixer output then from there a higher value resistor followed by some RF filtering to the grid of the AF pre amp valve, I could help negate losses by using an interstage transformer between the AF pre amp and the following stage.

I'm thinking of feeding the RF into the mixer via a cathode follower to get a low impedance match, question is, shunt or series fed for the RF input winding on a SBL1 mixer?

Lawrence.

6th Feb 2016, 10:16 am	#1
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Audio Transformer Trying to design a circuit, need to step up the output from a mixer (RF + LO in...AF out) The mixer output is designed to see a 50 ohm load, I need to get the AF output from the mixer to the grid of an AF amp, I'm thinking of a step up transformer, need max volts at the grid, frequency response not critical as the AF will be filtered down to 5Khz or less somewhere in the chain... Microphone transformer ? Cheaper alternative? Don't really want to go down the grounded grid route. Any thoughts folks. Lawrence.

6th Feb 2016, 10:26 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Audio Transformer An AF transformer connected straight onto the output of a mixer will have a dodgy impedance with all sorts of stray resonances at the RF and LO frequency. Most mixers give their best performance into a defined and resistive load. Use a diplexing filter after the mixer so that the input frequencies of the mixer and its LO range are connected to a load resistor to keep it happy, and lower frequencies go to the transformer. If you're building a direct-conversion receiver, the signal out of the mixer will be rather low, and low noise performance under the condition of the low impedance presented by the mixer is needed. This is an application where the transistor has a big advantage as well as saving an awkward to find transformer. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Feb 2016, 11:24 am	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Audio Transformer David, yes it's to do with a direct conversion receiver I want to build that I've rambled on about in other sections of the forum, don't ask me why but I want to do a hybrid job, no transistors in the RF/Mixer or AF, just valves on 12 volts HT, the hybrid bit comes in with the LO, I've order up a DDS VFO job. What I was thinking about was grafting a step up transformer into the output of a diplexer/filter of the type shown in Fig 4 here: https://www.arrl.org/files/file/Tech...df/9208019.pdf Can it be done or will it throw up other problems? Lawrence.

6th Feb 2016, 1:12 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Audio Transformer Hi Lawrence, Rick Campbell doesn't often miss a trick, but he missed on there. He's made the diplexer pass audio and reject everything above audio. Which means large inductor and capacitor values which means components which will be bad at RF. It would be better (and easier) to pitch the diplexer higher in frequency to split the audio off somewhat below the lowest RF frequency to be used. The purpose is to give the mixer a nice termination at those frequencies which will come out of it, and not to reflect them back in (which causes degraded mixer performance). David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Feb 2016, 1:45 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Audio Transformer Would this be better? (Fig. 10): http://www.robkalmeijer.nl/techniek/...991/04/page39/ Lawrence.

6th Feb 2016, 2:42 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Audio Transformer Lots! That's a very good article. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Feb 2016, 10:54 am	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Audio Transformer When I get some bits together I will try a resistive 50 ohm termination on the mixer output then from there a higher value resistor followed by some RF filtering to the grid of the AF pre amp valve, I could help negate losses by using an interstage transformer between the AF pre amp and the following stage. I'm thinking of feeding the RF into the mixer via a cathode follower to get a low impedance match, question is, shunt or series fed for the RF input winding on a SBL1 mixer? Lawrence.