Transistor hearing aid 1955.

[FERNSEH]

An early application of transistors. From the November 12th 1955 Electrical and Trading the Amplivox transistor hearing aid.
What type of transistors will be employed in the amplifier?
By late 1955 they would have moved away from point contact types and early junction transistors will be in use.

DFWB.

[Craig Sawyers]

1955 is an early point in transistor development, only 8 years after its invention. Assuming the hearing aid was developed during 1954, Bell had patented and were making diffused base transistors.

Sony introduced the first transistor radio in 1955, so predictable devices were certainly available then.

Craig

[G6ONEDave]

Given the size of the case, I would suggest Mullard OC65, OC66 or OC34 types. Larger hearing aids used Mullard OC70, OC71. Some manufacturers used GE and Raytheon types.

Dave

[ms660]

Originally Posted by Craig Sawyers

Sony introduced the first transistor radio in 1955, so predictable devices were certainly available then.

Tell Wiki:

https://en.wikipedia.org/wiki/Transistor_radio

Lawrence.

[Craig Sawyers]

Well it was not strictly the very first - but several companies worldwide were manufacturing transistor radios are around the same time.

[ms660]

Yes but it wasn't the first transistor radio, that was the Regency TR1.

Sony had a first with their TFM-151 radio.

Lawrence.

[paulsherwin]

The OC70/OC71/OC72 were introduced in 1954 so this aid was probably designed around those. Amplivox were an established British hearing aid company.

[Craig Sawyers]

They are often mounted horizontally held in place with specially designed plastic clips to keep the profile of the product low - as looks to be the place with the hearing aid.

Craig

[paulsherwin]

I wonder how the 'AVC model' worked - perhaps just a couple of diodes acting as a clipping limiter.

[G6Tanuki]

I guess you could do audio AVC by rectifying the output and using it to bias back a preceeding stage, just like you would in the IF stages of a transistor radio. Though the variation in the working point of the transistor could increase distortion, classic bipolar junction transistors are not really variable Mu in the way a tetrode or pentode valve can be.

[pmmunro]

The specification and prices are worth comparing to what would be offered today.

It's now very rare for technical detail of the performance for a device like this to be available with consumer products; perhaps it was meant for audiologists?

Using the Office for National Statistics price index, 1955 prices should be multipled by 47.25 times to give a comparitive price today, giving 45 and 50 guineas as £1580.50 and £1756. At least there was no purchase tax. How many people could have afforded to pay around 18 months wages for a skilled technician?

Apparently people pay typically £2000 for privately supplied hearing aids and often buy two. It's probably safe to assume that hearing aids are made in millions and can be adjusted to the user, they don't need to be individually made, so is this price justified?

It puts the price of ear defenders into perspective!

PMM

[G.Castle]

Mullard made special devices for hearing aid use: miniature types OC 57, 58, 59, and 60, though there were also miniature SO2 cased versions of the standard familiar range, a kind of glass version of the TO1, popular in pocket radios, denoted by the suffix 'M'.

The AGC system used in early hearing aid amplifiers was often a transistor used to control the capacitive emitter bypass of an early amplifier stage, a crude kind of level compressor.

Andrew Wylie has some information on the early germanium hearing aid transistors made by Phillips/Mullard/Valvo/Amperex.

http://www.wylie.org.uk/technology/s...rd/Mullard.htm

[Paul_RK]

Originally Posted by pmmunro

Using the Office for National Statistics price index, 1955 prices should be multipled by 47.25 times to give a comparitive price today, giving 45 and 50 guineas as £1580.50 and £1756. At least there was no purchase tax. How many people could have afforded to pay around 18 months wages for a skilled technician?

I think you're factoring inflation in twice there: around £1000 a year was good pay for a skilled technician in 1955, but it certainly isn't now! 45 to 50 guineas in '55 was in the ballpark of the price of a small TV, and around three weeks' wages for our skilled technician rather than 18 months' worth.

Paul

[paulsherwin]

Hearing aids have always been expensive (and still are). There's no real competitive pressure in the market, and customers tend to buy whatever their (private) audiologists advise. Most private medical technology is sold like this, notably heart pacemakers.

I imagine Philips/Mullard were keen to encourage hearing aid manufacturers to switch to transistor designs, as their high margin products could cover the high production costs of 1950s transistors while not being very technically demanding.