Not bad at all for a 40+ year old amp. Probably a great little headphone amp too. Radio Shack had all the coolest stuff back in the 70's. I miss those days.
I'm still using a SA-155 I have had since before 1995. It's been perfect for something on my PC desk to drive non-powered speakers from a choice of inputs, including transferring from my vinyl collection to digital because it inputs phono too. A reliable and attractive replacement would be north of $100 where I live. Only lately I think it's developed interference noise, so I took it "off line" but I won't throw it out. I'll have a poke at it again and if it is bad, at least it will provide a ready made and attractive case to drop in totally new boards and keep as much of the front and rear hardware as I can. Visually It makes the SA-100B here look like a dog, LOL. Realistic provided a great series of priced to sell products that got the job done; the same way Heathkit was one of my go-to stores back in the dark ages.
It's kind of stunning to think of all the man-hours that used to go into designing and hand-building one- or two-watt amps back then. And I think Radio Shack sold more of them than anyone. I had an SA-102 myself.
The SA-102 kicks ass, loud enough to annoy the neighbours in the units complex where I used to live, the speakers were 6" Matsushita from old TV in homemade cabs and the SPL 100dB
This is why I look at the specs and buy big wattage is the amount of clean headroom before it collapses. The guys in my band had 50 watt amps for their guitars and they laughed because I had 440 watts yet they always wondered why they couldn't get a good clean signal. They thought I just wanted to downed them out until I showed them when you want clean, you can't push them 50 watt amps and remain clean... They all went out and purchased the same amps I have. I never exceeded them in wattage anyway.
Interestingly, back then RS rated it at 10 watts of "music power". A little optimistic, I'd say. They also said 1% distortion without any qualifying parameters. Yes, I'd really like to see you do some testing on a 100C - maybe someday, I hope.
Hi John, just a thought...could that amplifier have been made for a 4 ohm load. Might give you 2-3 watts into such a load. Probably best not to tone test it however, those output transistors look very small!
Hello, I have one and want to recap. Do you have a recommendation to buy caps and info on parts needed? New to this and would really appreciate any info. Thank you for your time.
Hello! Recently I picked up a similar model amplifier, the Realistic SA-100C. And I feel like I should probably replace the caps on mine as well. Now from what i know you are supposed to match the capacitance when replacing the caps. Why do you increase these values (470 -> 1000, 1000 -> 3300, etc)?
The 100C is a more modern design using silicon transistors instead of germanium. It has less caps. I increased the supply filtering as 1,000uf is a bit low for an audio amp. The 470uf caps were supply bypass for the preamp stage. Increasing them was not really necessary, but I had a bunch of those 1000uf ones on hand. I'd leave the others at the same value unless you can determine that they should be changed.
I have this amp and would like to recap it. It will be my first attempt at something like this. Would you happen to have a copy of the schematic or a list of the values of the original capacitors? I have been unable to find this online.
john you think it's possible to build a protection "general purpose ( to protect the output power transistors ) ? i know every amplifier need to calculate severals parametre ,one amplifier is not another but ...? because the darlington are so fragile ! if you short circuit the output , power transitors and drivers burn immediatly :(
Wondering about the transistors or diodes mounted to the tops of the transformers. Just a heatsink for the driver? Or part of thermal management/bias? Doesn't seem like the transformers would warm in an amp that small.
Those are the transistors that drive the transformer. It is a class A type stage that gets a little warm. Using the transformer is just a convenient mounting point for a heat sink. I guess germanium transistors are heat sensitive, therefore, keeping them cool is important.
if you want to learn and also to build an amplifier just check datasheet from TDA 2003 ,i maked a stereo amplifier with two TDA 2003 it work very well and so easy to make ;)
i'm at a number of 20 amplifiers diy and i use a maximum compo,ements from the 80s like couple of darlington (TIP142/147) or other 3055 with larger capacitors and resistors ^^ so easy to make and of course ,very easy to fix
Now-a-days you can buy 10 modules like PAM8043 3W stereo amps for less than $2 !!! goo.gl/CzJxJr That's twice the power of this Radio Shack thingy. I am more interested in how they used to do it.
No. The standard fare 1000uf caps should have an ESR of only a few tens of milliohms. Driving a relatively much greater impedance of 8 ohms, I don't see the benefit.
JohnAudioTech is this wav file shared or could you upload it somewhere? I do not have a sign generator and this would help doing this type of measurements you do!
Not bad at all for a 40+ year old amp. Probably a great little headphone amp too. Radio Shack had all the coolest stuff back in the 70's. I miss those days.
Now, the uncle-amp is reconditioned to its full performance. I'm happy. (Granny-amps are for tube-amps)
Cheers from Indonesia
I just like the look of the 70-80 amps. Great Video.
I'm still using a SA-155 I have had since before 1995. It's been perfect for something on my PC desk to drive non-powered speakers from a choice of inputs, including transferring from my vinyl collection to digital because it inputs phono too. A reliable and attractive replacement would be north of $100 where I live.
Only lately I think it's developed interference noise, so I took it "off line" but I won't throw it out. I'll have a poke at it again and if it is bad, at least it will provide a ready made and attractive case to drop in totally new boards and keep as much of the front and rear hardware as I can. Visually It makes the SA-100B here look like a dog, LOL.
Realistic provided a great series of priced to sell products that got the job done; the same way Heathkit was one of my go-to stores back in the dark ages.
It's kind of stunning to think of all the man-hours that used to go into designing and hand-building one- or two-watt amps back then. And I think Radio Shack sold more of them than anyone. I had an SA-102 myself.
The SA-102 kicks ass, loud enough to annoy the neighbours in the units complex where I used to live, the speakers were 6" Matsushita from old TV in homemade cabs and the SPL 100dB
This is a nice little vintage amp. Very professional recap job!
that amp sounds so much better than many things made in recent years
Thanks John. I appreciate your videos.
This is why I look at the specs and buy big wattage is the amount of clean headroom before it collapses. The guys in my band had 50 watt amps for their guitars and they laughed because I had 440 watts yet they always wondered why they couldn't get a good clean signal. They thought I just wanted to downed them out until I showed them when you want clean, you can't push them 50 watt amps and remain clean... They all went out and purchased the same amps I have. I never exceeded them in wattage anyway.
Interestingly, back then RS rated it at 10 watts of "music power". A little optimistic, I'd say. They also said 1% distortion without any qualifying parameters. Yes, I'd really like to see you do some testing on a 100C - maybe someday, I hope.
3.37v RMS is 9.5v pk-pk which gives 11.3 watts peak into 8 ohms, so it does actually stack up with the silly "music power" rating.
You make very good videos, good upgrade.
Hi John, just a thought...could that amplifier have been made for a 4 ohm load. Might give you 2-3 watts into such a load. Probably best not to tone test it however, those output transistors look very small!
Nice job John. Cute little amp.
Hello, I have one and want to recap. Do you have a recommendation to buy caps and info on parts needed? New to this and would really appreciate any info. Thank you for your time.
Hello! Recently I picked up a similar model amplifier, the Realistic SA-100C. And I feel like I should probably replace the caps on mine as well. Now from what i know you are supposed to match the capacitance when replacing the caps. Why do you increase these values (470 -> 1000, 1000 -> 3300, etc)?
The 100C is a more modern design using silicon transistors instead of germanium. It has less caps. I increased the supply filtering as 1,000uf is a bit low for an audio amp. The 470uf caps were supply bypass for the preamp stage. Increasing them was not really necessary, but I had a bunch of those 1000uf ones on hand. I'd leave the others at the same value unless you can determine that they should be changed.
I have this amp and would like to recap it. It will be my first attempt at something like this.
Would you happen to have a copy of the schematic or a list of the values of the original capacitors?
I have been unable to find this online.
The Hifi engine site might have a schematic (you have to register, but it is free).
i have a 1978 amplifier sony and his couple of capacitors (6800microfaradX2) work allways fine ! respect for those old parts ;)
Aside from the bad capacitors I changed in the first video, I don't think any of these were bad. Oh well, it has a new set of caps.
It lives!
john you think it's possible to build a protection "general purpose ( to protect the output power transistors ) ? i know every amplifier need to calculate severals parametre ,one amplifier is not another but ...?
because the darlington are so fragile ! if you short circuit the output , power transitors and drivers burn immediatly :(
Always enjoy your videos, John. Disappointed though, that you don’t let us see your desoldering technique.😀
I just use desoldering braid. It was a little difficult to get the parts off because they bent the leads over tight against the board.
Wondering about the transistors or diodes mounted to the tops of the transformers. Just a heatsink for the driver? Or part of thermal management/bias? Doesn't seem like the transformers would warm in an amp that small.
Those are the transistors that drive the transformer. It is a class A type stage that gets a little warm. Using the transformer is just a convenient mounting point for a heat sink. I guess germanium transistors are heat sensitive, therefore, keeping them cool is important.
I wish you would do schematic analysis on your videos. Would learn a lot more about Audio circuit design schemes.
I plan a video on how these old amps work. I talked a bit on the schematic of this amp in the first video.
if you want to learn and also to build an amplifier just check datasheet from TDA 2003 ,i maked a stereo amplifier with two TDA 2003 it work very well and so easy to make ;)
i'm at a number of 20 amplifiers diy and i use a maximum compo,ements from the 80s like couple of darlington (TIP142/147) or other 3055 with larger capacitors and resistors ^^ so easy to make and of course ,very easy to fix
Now-a-days you can buy 10 modules like PAM8043 3W stereo amps for less than $2 !!!
goo.gl/CzJxJr
That's twice the power of this Radio Shack thingy.
I am more interested in how they used to do it.
module PAM8043 use for bluetooth speakers it's a class D amplifier very small without any heatsink !
that sounds MUCH cleaner in both channels
Did you spec the ESR for the output caps?
No. The standard fare 1000uf caps should have an ESR of only a few tens of milliohms. Driving a relatively much greater impedance of 8 ohms, I don't see the benefit.
It was a sinle ended amp,i gues?
Hello! Do you use a signal gen for that sweep?
I made a 20 to 20K sweep .wav for my audio player.
JohnAudioTech is this wav file shared or could you upload it somewhere? I do not have a sign generator and this would help doing this type of measurements you do!
Yes they are available online. I am adding some more files and will make it available soon in an upcoming video.
do not use different value caps from original it can screw stuff up
Nice!
I would piss off the stupid loudness and desolder the caps at one end, I hate the tone change at over half way giving the midrange lump