January 18, 2016 - 08:02 am|
|Hi Steve, Thanks for the answer and I have another 3 questions please if you don't mind.|
1. Dr. H. C. Lin's power amp. circuit (1959) has R3 as 47K, no capacitor C302. EIC split this into 2 x 27K - R302 & R303 with C302, presumably to remove this as an ac signal path back into Q301 base, right?
How it is determined that it is 1/2 and 1/2 rather than some other ratio?
2. I gather Q301 DC Bias = ratio of R304:R302+R303, off the centre rail that settles to +Vcc/2. Why not take a resistor divide from +Vcc to ground for Q301 bias? Not so good?
3. To effect bootstrapping across R306, R305 is required. How is the 1K decided on please?
I assume the 50V 47 mF on the +Vcc rail removes ac from the DC supply, right?
January 18, 2016 - 07:54 am|
|Hi Les, Thank you for your kind message and insightful question. You are absolutely correct, that there will be some DC feedforward current through R312 to the output. However, it will be very small in practice. Without a speaker connected, about 54mV will appear across R16, due to the divide ratio between it and R312. If a connected "8ohm" speaker has, say, 6ohms DC resistance, the DC voltage at the output will drop to around 0.3mV. Hence, the DC is insignificant. It's an interesting observation though.|
Congratulations on your self-education program. From your question, it seems to be working! :) Best wishes with your studies in EM!
January 18, 2016 - 07:51 am|
May I say what an excellent description of the E.I.C. equipment circuits you have provided.
I myself, out of writing software and industrial consulting, decided to buy > 50 textbooks /workbooks on electronics, electrical engineering and calculus (including GE's GlowLamp Manual 1963 - I like negative resistance devices) and have been teaching myself electronics / elec.eng. for the past few years.
As a test of skill and before I move onto Electromagnetics, I have been studying your description of the EICO Cortina Amplifier circuits, particularly the power amp. stage, along with Dr. H. C. Lin's RCA 10 Watt power amp.
If I may ask . . . Re "The DC output voltage (ideally half the supply voltage) is set by feedback through R302 and R303". Isn't a DC voltage put across R16 load via the feedback resistor R312? = feedforward?
As a result of the DC Bias on Q301, set up by the ratio of R304 to R302+R303 x centrerail DC voltage?
Regards ... Les, Melbourne, Australia;
August 17, 2015 - 04:34 pm|
April 17, 2014 - 10:24 am|
|Hi Ann, I really appreciate your kind message. It's so nice to hear that you enjoyed the Cortina. And it's especially nice to hear from a lady who appreciates such a thing. Alas, vintage audio enthusiasts are almost exclusively guys and one occasionally hears a lament about the fact that ladies aren't usually interested (as you note). Bravo, for your efforts researching amps and tuners. I hope that you find something which makes you as happy as the Cortina did. Thank you so much for writing. |
April 17, 2014 - 08:28 am|
|Hello, I just wanted to send a short note to let you know how much I enjoyed your article about the Eico Cortina 3070. What a thrill of nostalgia! Awwwww.... was my first reaction on seeing the photo. I had an Eico Cortina 3070, a gift from my Dad who was an engineer and built his own Heathkits and always had the latest and best in stereo for himself, from about 1970(?) until 1992, when I sold it to move to France. (I was afraid it wouldn't survive the journey, or the converted electrical current, etc. upon arrival here).|
Nobody ever snubbed my little Eico Cortina; in fact men loved it and treated it with great respect. ("Look, she has an Eico Cortina !"...) (...Women didn't care.) It gave great sound, on four speakers throughout my apartment, from Daniel Barenboim to Ted Nugent, and no trouble ever for all that time.
By the time I had my own apartment here in Paris and bought a "new" stereo system, I made the mistake of buying a used Akai. Boy did I hate that amp. It didn't hold a candle to the little Eico! It finally died, thank goodness, and I'm replacing it, so I've been reading a lot on the web about amps and tuners.
Your article made me realize that it's possible to "love" and to "hate" inanimate objects like hi-fi amplifiers. How I miss my little Eico! Enjoy yours and keep up the blog!