Speaker and amp matching?

SamyBoy13 · 30 Aug 2017 at 15:17

Hi there guys I recently got a Fisher Ca-224 amp and monitor audio br1 from a carboot sale from two different sellers. I paid 30 pounds is total for the set. However I haven't realised that the speakers are 6ohm whereas the amp is rated at 8 ohm only 2x35 rms.

I connected them up together and at 20% volume they sound impresive and loud enough with no distortion or hissing. However I'm worried that through not matching the impedancesearch, I will break either or both.

Any advise would be appreciated
Thanks Sam

Fisher CA-224
Integrated Stereo Amplifier 1986

Specifications
Power output: 35 watts per channel into 8Ω (stereo)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.5%

Damping factor: 20

Input sensitivity: 2.5mV (MM), 150mV (line)

Signal to noise ratio: 68dB (MM), 85dB (line)

Dimensions: 400 x 91 x 202mm

Weight: 4.1kg

Year: 1986

hornetstinger · 30 Aug 2017 at 15:19

You'll be fine. However 35W is not that powerful so just be careful. The amplifier should output about 40-50W into your 6ohm speakers

Being that old I'd get the capaciators replaced.

SamyBoy13 · 30 Aug 2017 at 15:32

Any ideas how much would that cost as I'm not able to do it myself. Also any risk associated with bad capacitors?

hornetstinger · 30 Aug 2017 at 19:33

Probably £75-£100. Capacitors do age over the years. I am not sure of specifics of negatives for old capacitors, worst case scenarios.

Kei · 30 Aug 2017 at 20:24

Haven't seen a fisher amp in years. They aren't all that common these days. They used to make some fine kit back in the 70's.

Absolute worst case scenario for knackered capacitors is a small bang and it no longer works and needs a lot of new parts, however this is pretty uncommon. Generally, there is a deterioration in the quality of the amplified passband, be it increased roll off of high frequencies, bumps in the LF response or simply more EM noise getting through. No idea on the cost of getting a repairer to do the work, but the parts themselves are generally quite cheap, even for quality brands so you'll be paying mainly for labour.

With that amp, 4 ohms is fine but due to the nature of the type of circuitry used (STK4152II), whilst you do get more power out due to the nature of increasing the current drawn and having a fixed voltage rail, the cost is the THD increasing from 0.5%. (at 4 ohms it would be around 1% so somewhere in between for 6) At nominal listening levels, this is unlikely to be noticeable so don't worry about it. Only thing that would cause issues would be having the volume very high, with lots of high level low frequencies. Avoid turning the bass up.

To be fair, for the cost, I'd just enjoy it for what it is.

JasonM · 30 Aug 2017 at 20:30

6ohm speakers into 8ohm amp won't matter, only thing to say is once you get above 60% of the amp dial, just place your hand over amp and make sure it's not overheating.

Re capacitors they do age, however I have amps from early 1990's still in regular use. What I would do is open amp and check no capacitors are leaking, otherwise your good to use the amp.

SamyBoy13 · 30 Aug 2017 at 21:49

I opened it up and this is how it looks. When I connected them to br1 to see if it works. I was surprised as the denon dab 37 I got roughly 1 year ago, makes hissing noise all the time and this old amp dead is silent.

Sometimes I wished I knew how to fix electronics.

hornetstinger · 30 Aug 2017 at 22:05

You could just replace those two large capacitors. That's the PSU caps, it won't cost much.

SamyBoy13 · 31 Aug 2017 at 01:40

Okay. Out of interest what is the worst case scenario when a power capacitor decides to blow up. Also if other smaller capacitors blow up could they potentially damage br1?

I don't mind service the entire amp but I'm not sure how much the amp is worth. It's only 2x35 rms into 8 ohm so probably it isn't work that much. Although I'm not sure.

Further, in all fairness the amp costed only 5 pounds and speakers 25 so I don't want to pay really 70-100 pounds to replace most of the caps as for additional 50 I can get new denon dab 40 with 3yrs warranty and peace of mind that if it breaks it will be fixed free of charge.

Kei · 31 Aug 2017 at 18:46

Personally, they look ok. I've worked on amps of a similar vintage and older and only some of them have benefited from having any of the capacitors replaced. (usually only those that had capacitors located right next to things that got very hot)

This amp, only 4 capacitors were out of spec, which were the ones living next to the 1W resistors that get hot and have discoloured the pcb. The giant blue caps are the originals as I cannot find any suitable replacements as the lead spacing and shape of the caps is unique. The amp is from 1984 and still working beautifully.

Same story here, very hot zener diode and the 2W resistor next to it cooked the capacitors around it. (took the zener out as it was no longer needed after converting the lamps to LEDs) This amp is also from around 1984.

SamyBoy13 said:
Okay. Out of interest what is the worst case scenario when a power capacitor decides to blow up. Also if other smaller capacitors blow up could they potentially damage br1?

Any direct coupled amplifier can damage your speakers if it does go wrong. The amp has fuses on the outputs which may protect the woofers, but the tweeters would probably be toast before the fuses blew. As I said before, it is unlikely that it will go wrong. I replaced all of the capacitors in my dad's Leak 2200 from 1974 five years ago (38 years old at the time) and the improvement in sound was small. The speakers on the other hand had a huge improvement as the crossover caps were thoroughly knackered.

As to the cost of doing the work. Those large 3300uF 50V capacitors are going to be the most expensive parts and even they are dirt cheap for high quality brands. For the cost of the amp, you could use it as practice for learning how to solder.

SamyBoy13 · 31 Aug 2017 at 23:26

Thanks a lot for your post KEI. I may give it a try. Any tips on how to test transistors, resistors, caps etc.? The only thing I have is a multi meter and soldering iron so not sure if I will need anything else, except the parts.

The only other issue I noticed that only CD red and white input works as other make makes crackling noise form speakers. Otherwise, the speakers play exactly the same as connected to my denon 37dab (At least I cant hear any difference with BR1s)

I have to admit I love the style of this amp, even though it's older than me

Kei · 1 Sep 2017 at 12:23

The crackling issue might well be the input selector switches. They may well need some contact cleaner and more regular use. The input labelled "Phono" located on the far left side on the back is only for turntables as it is designed for very low level signals which need more gain and equalisation.

Testing components generally requires them to be removed from the circuit that they are in. Resistors just measure and then you can check what you measure against the value it should be given by the colour bands. Capacitors can be more difficult to measure properly and usually need a decent LCR meter to do the job correctly.

This site has some good information on it regarding electronics. I'd avoid learning basic electronics on an amplifier though as they operate with fairly high DC voltages which can be lethal if you go prodding the wrong thing whilst it's powered. Heck some larger capacitors can store enough energy and discharge it later that can kill. Obviously I'm not trying to dissuade you from learning, just advising that caution is wise. Learning on lower powered devices is a safer place to start.