Golborne Vintage Radio

Hi
My current project is making an active loudspeaker, but more on that later.

I was recently given some Sinclair Z30's and I have got one working which will be used to power the speaker.

In the Project 60 manual Sinclair give a circuit for tome controls to go with the amp which give a 15db cut and boost to the treble and bass. But there is a considerable loss of signal when using this type of tone controls.
[attachment=18490]

Is it possible to alter the values of the circuit to reduce the losses and to just have say 6db cut and boost to the bass and treble.
Can someone please work out suitable values.

Thanks
Mike

The usual way to do it without loss of gain is to use pretty well the same circuit in the negative feedback line.
Bass and treble controls swap around because the effect is inverted due to the fact that the feedback is negative.

For an example of an amp that has a passive Baxandall with only +/-6dB of range, check out the Rotel RA-930.

https://elektrotanya.com/rotel_ra-930ax....nload.html

The insertion loss from this circuit is 8dB. That's likely as good as you'd get with a passive circuit. For that and various other reasons, I much prefer active, and good audio op-amps are cheap.

One trap: the input impedance of these circuits (active or passive) vary with frequency - you might need to look at what precedes them and consider adding buffer stage - just a single op-amp. Of course, if you're doing that, then that could be used to provide some of the gain lost in the network...

I'd expect a TDA2030 or TDA2050 or LM1875 would give better results than that Sinclair module, but if you already have them...

Did the Z30 ever work? Sinclair designs from that era were more than slightly dubious.

Mike, what you are proposing is not what I would call an 'active' speaker - just a passive speaker driven by an amplifier with a wide range tone control.

The problem with audio, given decent quality speakers is the compromise between size and bas response and an active speaker can help here.

In the 70s, Philips introduced an active speaker which had a piezo accelerometer incorporated in the cone of the woofer to provide feedback proportional to movement of the cone which could be used to correct the drive power with frequency to obtain an extended level bass response.

The second UK issue of Elektor included a design to replicate this function in a way that didn't require the special Philips woofer but instead used a carefully controlled level of positive (yes!) feedback to achieve the same result.

Elektor demonstrated the prototype on their stand at an exhibition in either Earls Court or Olympia that I went to and the results were amazing! I can let you have a pdf copy of that issue if you are interested. You can still use your Sinclair amps if they do, on fact, work - the article describe the additional electronics that precede the amplifier inputs.

I know some Elektor designs can be found on the internet but I think they are rather fussy about copyright, so I'll put a copy of the file (37MB!) in my Dropbox and send you the link if you want it.

There has also been work done with a speaker in a "motional feedback bridge". This led on from the Philips work that also had applications in ultrasonics.
The bridge output was proportional to the speakers motion; error correcting feedback was then applied to clean up the response.

Ed

The term "active loudspeaker" might be something as simple as a speaker with an amp in the same box. Most computer speakers are like this and it seems entirely sensible. both for this sort of speaker and also for high quality designs. The BBC LS5/8 is a well known example. This used the 2 channels of a Quad 405 amp to drive the woofer and tweeter of a high quality monitor speaker. There are obvious advantages in doing the crossover at low level. Also sending the signal to the speaker at line level (balanced in a professional design) avoids any issues with speaker cables.

I'm am little surprised that motional feedback or something similar hasn't become more common. Electronics are cheap and it could be a good way of getting high quality audio out of a small and cheap speaker.

Many years ago I dimly remember reading about feeding square (square-ish?) waves into different loudspeakers and looking at the result using a measurement grade microphone. Apparently the results looked horrible except on the Quad ESLs. I think this was before the ESL63 was around.

Hi
Thanks for the link to the Rotel amp Mark.

Yes the Sinclair Z30 did work but in many cases there were problems with failures.
I probably spent more on building my first "HIFI" amp than if I had gone and bought a Leak Stereo 30. The main problems I think were associated with trying to push them to the limits of the specs. claimed by Sinclair. I believe it would have been more accurate for Sinclair to have named it a Z20 and stated against all the specs "or".
For instance rather than 20 watts at 0.001% distortion it should have said 20 watts or 0.001% distortion.
I was a bit naive and not as informed as I am now. But I did get my Project 60 amp working reliably, eventually and even won a 1st handycraft prize at school when I showed it.
The methods of construction and heat dissipation from the output pair were not good. With 2 heat transfer joints and maybe 3 joints for the heat to transfer from the transistor, to the final heat sink. Once an output transistor had blown it tended to have a cascade effect through the amp.
They become more reliable if the output transistors were removed from the small aluminium mounting bracket and mounted directly (with mica washers etc.) on to a very large finned heat sink, and preferably uprated. On my final version of my amp after buying two new Z50 amps I changed the transistors to 2N3055 types, I found these to be fairly bullet proof.

This example I am playing with had already had uprated output transistors fitted (TIP41A). I have checked all of the other transistors and replaced some damaged components and very leaky electrolytics, the amp is now working reliably.
But I only managed to squeeze about 16 watts out of it at the rated voltage of 35v into an 8 ohm dummy load. The distortion was terrific, good enough for a guitar amp Sam said, it rendered the music content indistinguishable.

This amp however is going to have a fairly easy time of it supplying no more than 2 watts to a 10 inch Goodmans speaker. Supplied from a 9.5v 1.5A (in reality more like 14 volts) Cannon power supply.
I have not been striving for anything technically brilliant or accurate hifi, just having fun and making use of any old pieces of junk.

I suppose now is the time to introduce the speaker.
It came thrown in for free with a whole lot of KB ephemera, pamphlets, user instructions etc. and a unique original KB BR40T prototype radio.
Here is the speaker more or less as I got it, with having had just a few taps with a mallet, to help separate the bits.
The KB fleur-de-lis fretwork had a few breaks in it which were fairly easy to repair with some of the de-laminated oak ply from the base.
The rest of the pieces were all separated and all old lacquer removed to leave a natural oak finish which was thoroughly rubbed down with finer and finer sand paper and finished with fine wire wool.
After gluing the joint in the top panel back together with biscuits, the cabinet was then reassembled.
After reassembly and a little tidying up it was given a very light stain, and then waxed and left with the natural look finish.
This was all started way back early last year, and I was wondering what to do with it, then the offer of the Sinclair stuff came up.

Here are some photos of the work done last year.
[attachment=18499] [attachment=18501]
[attachment=18500] [attachment=18504]
[attachment=18502] [attachment=18503]

Mike

Hi Terry
I think what you mentioned was a different type of more advanced active speaker. Active in the scene of Active Speaker simply means it is not a Passive speaker. It has active components such as an amplifier and PSU so that no additional amplifier is required to drive it.
https://www.whathifi.com/promoted/just-w...ers-anyway
What you mentioned is something more advanced and is over and above a basic active speaker.

Mike

Hi Mike, the bay lists a pre amp kit using NE5532s with bass and treble controls for under 4 quid. Really nice blue PCB dead easy to build.  Even has + and - regulators on board. Various power amp modules listed too at very low prices. I dont really like the idea of using Chinese bits but at least this stuff doesnt send your commands back to base!

Peter