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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 31st Oct 2014, 4:41 pm   #1
stevehertz
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Default Hacker Mayflower tone correction

In the thread that I started previously concerning the restoration of my Hacker Mayflower RV20, towards the end I commented on the poor bass response of the set. In fact I’m not the first person to comment on this, there is a previous thread entitled ‘Hacker Mayflower sound quality’ that attracted a fair amount of comment and discussion. In my thread, Mark Hennessy suggested that I insert a passive shelf network between the radio output socket and the gram input socket. Well, I’ve been experimenting and I have now arrived at a situation that I’m happy with and the sound of the set is much improved.

Basically I built a small R/C network across a pair of phono sockets mounted in a tiny die cast box measuring just 51x36x31mm (2 inches long). The box straps onto the back of the Mayflower and using a tie wrap and uses a short stereo phono lead to connect to the aforementioned sockets. There are zero modifications to the set itself.

Background. The HM RV20 has a great audio output circuit; push-pull, ultra linear configured, with EL84 output bottles. The sound is high on ‘fidelity’ yet conversely, uninspiring and frankly, boxy, and lacking in bass. The situation is not helped by the fact that the set – for all its technical sophistication – has just a single tone control, a treble control, and even that is mounted on the rear of the chassis. It’s my view that Hacker wanted to design a smart, neat, clean looking set in the modern idiom. In fact there are no knobs on the front of the set and just a tuning control at one end and the on/off/volume at the other end. Compare that with its contemporaries by Grundig, Bush and many others who included bass and treble controls, and some – especially Grundig – also included intermediate frequency tone controls to finely tune the set to one’s satisfaction or, to counter driver abnormalities. These sets also used multiple speakers including tweeters and side mounted drivers to spread the sound and make it ‘big’, inspiring and 3D-like. Ok, I’m not saying that Hacker were obliged to go down that multi-driver route, but they could have at least included a bass tone control that, in conjunction with the treble control could be tweaked to counter the ‘turtle’s back’ frequency response of that 10x6 speaker.

The shelf circuit used comprises two resistors and a capacitor. Referring to the diagram included, the values are as follows. R1 270K, R2 100K, and C1 is 6.8nF. I have experimented with those values, and after over a week of listening to that particular configuration, the sound of the set now meets my expectations. Gone is the honky, boxy, nasal mid range and the bass is so much better too. The shelf works best with the treble control up full. This also has the effect of helping to reduce the mid range honk of the speaker while simultaneously lifting the higher frequencies that are not so well catered for with a 10x6 inch driver. If someone wanted to go for more of a halfway house situation then I’d suggest R1 to be 180K. Note that there’s an added benefit in that there’s now more ‘spread’ on the volume control where it all used to come in at the bottom end of the pot on strong stations. One more point, the phono plug that is used in the gram input socket must be one of the old style ones with a long central pin. This is because this pin operates a switch that disconnects the set’s radio section in order for the socket to work as required. A modern style phono plug with a short pin will not work, it does not operate the switch. I was lucky, I just happened to have one old style phono plug at hand.

Finally, my mentor on the Hacker Mayflower, Mark Hennessy must take credit for the shelf design and general technical advice and ‘hand holding’. I – as a musician and long time studio technician – just provided the ears and fine tuned the values of the network.
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Old 31st Oct 2014, 7:12 pm   #2
Nickthedentist
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Default Re: Hacker Mayflower tone correction

I love that way you've done this with no mods to the set.

This is such a simple contraption, that I will definitely try it one day.

I find my Mayflower II sounds much better stood on a revolving bookcase some way away from the walls. On a shelf near a wall, it sounds very ordinary for what it is.

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Old 31st Oct 2014, 10:06 pm   #3
stevehertz
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Default Re: Hacker Mayflower tone correction

It occurred to me that the speaker in these sets could age in different ways depending on how the set has been used and stored. If I can explain my thinking. On mine, the suspension - the corrugated ring around the speaker cone - is in good condition, and when you try to gently move the cone you feel stiff resistance. Just supposition, but I'm thinking that under certain storage conditions - maybe very dry and warm/hot - the cone will stiffen and 'set' with age (not with USE, just storage), in other conditions, it may soften. Similarly, if the speaker cone has not been driven hard you would also expect the the surround to remain stiff. On the other hand if the speaker has been subjected to long term loud(ish) usage, then unless the suspension is actually damaged, it will be more compliant and able to make larger excursions and thus capable of better bass. There's a lot of variables going on.

So, given these two basic variables; 1) storage conditions, namely heat, humidity, light etc, and 2) usage, namely volume, programme material, and tone setting, then as we stand now in 2014, the speakers in different examples of these sets will have different frequency response curves as a result. The R/C network shelf that I arrived at is optimised for the speaker in my set. But saying that, I'm pretty sure that those values would benefit most examples, or at least, be a good starting point. But basically, if you want to experiment, increasing the value of R1 creates a deeper shelf (the bass is more pronounced compared to mids and highs), and reducing the value of C allows the lifted or 'boosted' bass to extend towards the lower mid frequencies. So the former affects the bass level, and the latter affects the frequency at which it kicks in.
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Old 1st Nov 2014, 1:45 pm   #4
stevehertz
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Default Re: Hacker Mayflower tone correction

Here's some more photos showing the finished job. The first shows the tone correction module strapped to the back of the set and connected using a stereo phono lead. The second shot shows the difference between new and old style phono plugs. The latter has a longer centre pin that is needed to operate the switch within the gram input socket.
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