SA-300 vs 300B
- Thread starter squeel
- Start date
Register to hide this ad
The center channel jack is the only difference. I've never measured the power output of an SA-300, but the difference could be as simple as a typo, or a different rating system being used (RMS versus IHFM).
Dave
Dave
Yes, I know this is an old thread. I am working on a Fisher SA-300 and SA-300-B and was wondering about what the differences may be.
It seems that some (earlier?) SA-300 amplifiers did not have a center channel jack while some (later?) SA-300 amplifiers did have the center jack. My SA-300 does have the center channel jack.
Re power: There is a sales brochure on hi fi engine that states: "Power Rating 90 watts of music power (new IHFM standard); 45 watts per channel. Conservatively rated at over 35 watts R.M.S. per channel."
The fundamental difference between the SA-300 and SA-300-B could be that the SA-300-B has a "Heavy cased power transformer low flux density and heavy copper strap to prevent stray fields." My SA-300 does not have the copper strap on the power transformer while my SA-300-B does.
Are you sure? Use of a hum strap was standard on all Fisher power transformers. On the SA-300 (mine is an early SA-300 model), the copper strap is on the top of the transformer, and is also painted black, so it is not immediately obvious without closer examination.
Besides the addition of the Center Channel output jack, the B versions also eliminated the input high pass filter, but still retained the low pass filter of the original design. Otherwise, the SA-300 and SA-300B are identical.
From measured results using a 121 volt power source, a stock SA-300 or SA-300B is capable of producing 30 watts RMS from 20 Hz to 20 kHz, with both channels driven.
I hope this helps!
Dave
Besides the addition of the Center Channel output jack, the B versions also eliminated the input high pass filter, but still retained the low pass filter of the original design. Otherwise, the SA-300 and SA-300B are identical.
From measured results using a 121 volt power source, a stock SA-300 or SA-300B is capable of producing 30 watts RMS from 20 Hz to 20 kHz, with both channels driven.
I hope this helps!
Dave
No, I am not sure. I was speculating based on the two examples sitting on my test bench. My model SA-300 does not have the strap. My model SA-300-B does. Your added data point is that you have a SA-300 with the copper strap. So, this means that the difference between the SA-300 and SA-300-B is not the copper strap. It may be that the copper strap on my particular SA-300 fell off or was removed.
We also know that the difference is not the center channel output since I have a SA-300 with a center channel output, which would suggest that, at least, later versions of the SA-300 have the center channel output.
Maybe the only consistent difference is the high pass filter?
If you're certain that your power transformer does not have the strap on the top of the lams, then I would more wonder about the possibility of the transformer being a replacement. The strap is not something that can fall off, as it is a continuous strap that wraps around the coil form going under the end bells, but outside of the lams so it won't appear to the transformer as a shorted turn. Sometimes they are installed before paint is applied, and then end up being painted black along with the transformer when it is painted. Other times the transformer is painted first and then the strap is installed so you clearly see the strap when the transformer is finished. Invariably, the strap on almost all Fisher transformers is painted black making it less noticeable.
As for the center channel output, I would imagine that it was considered a feature of the B model, but that the actual start of installing that feature may not have coincided exactly with chassis silk screening or serial numbers to support the change. I do know there are a number of instances with Fisher where upcoming circuit changes started being installed before the actual serial number was reached that documented the change. For example, there the final build of the 80AZ boxcar amplifiers were actually built with the model 100 amplifier circuit, so sometimes, serial number changes don't exactly coincide with what's under the hood.
Since your 300 model does not have the strap, does it have any problem with hum? Fisher went to great lengths to minimize noise in that amplifier (in fact making it a selling point) through the use of a hum strap -- and -- using a chassis deck made out of aluminum. It can in fact have a hum and noise level that is 100 db below full power output -- but boy oh boy. In stock form, they are space heaters for sure!
Dave
As for the center channel output, I would imagine that it was considered a feature of the B model, but that the actual start of installing that feature may not have coincided exactly with chassis silk screening or serial numbers to support the change. I do know there are a number of instances with Fisher where upcoming circuit changes started being installed before the actual serial number was reached that documented the change. For example, there the final build of the 80AZ boxcar amplifiers were actually built with the model 100 amplifier circuit, so sometimes, serial number changes don't exactly coincide with what's under the hood.
Since your 300 model does not have the strap, does it have any problem with hum? Fisher went to great lengths to minimize noise in that amplifier (in fact making it a selling point) through the use of a hum strap -- and -- using a chassis deck made out of aluminum. It can in fact have a hum and noise level that is 100 db below full power output -- but boy oh boy. In stock form, they are space heaters for sure!
Dave
tcdriver...
The difference between model SA300 and SA300B has always up in the air and I've only seen a schematic for the SA300B. Since you have both versions on your bench, any chance you can snap some pictures from underneath?
I have a SA300B in use but also own a SA300 (a spare currently in storage). I'm a little interested now and will try to take some images of my SA300 this weekend.
It's been awhile since I looked but I'm pretty sure my SA300 does not have a center channel. I'm also pretty sure the the AC balance switch is not on the same side as with my SA300B.
The difference between model SA300 and SA300B has always up in the air and I've only seen a schematic for the SA300B. Since you have both versions on your bench, any chance you can snap some pictures from underneath?
I have a SA300B in use but also own a SA300 (a spare currently in storage). I'm a little interested now and will try to take some images of my SA300 this weekend.
It's been awhile since I looked but I'm pretty sure my SA300 does not have a center channel. I'm also pretty sure the the AC balance switch is not on the same side as with my SA300B.
I am posting two pictures of the two Fisher amplifier's transformers. The first picture is a blurry SA-330-B with the strap, that Dave has described, clearly in place. The button on top is a temperature sense probe that I am using to measure the temperature of the transformer.
The second picture is my SA-300. Although not clear from the picture, the size and shape of the transformer is identical to the SA-300-B. Note: It looks like there was something (a piece of copper perhaps) that was glued on top of the transformer at one time. The outline and glue residue is pretty clear.
That may be the explanation for why my SA-300 has the center channel output.
I will have to get back to you on the hum question. I just pulled this out of "the barn" where it has been sitting for the last 30+ years. I am going to have to replace some capacitors before I will be able to power it up. And, yes, my SA-300-B, that is running, makes a pretty good space heater.
Last edited:
At some point I will take some photos, however; both units have had some modifications and/or parts replaced, so I am not sure how good the comparison can be.
Yes, I would like to see the photos of your units, if you can post them.
Interesting! By all appearances however, it's a virtual certainty that the strap was in place when the transformer was manufactured. Can't imagine why anyone would want to remove it. The good news is that it can surely be replaced, although the transformer will have to be disconnected, removed, and disassembled (end bells taken off) to do it. The process would be no more simple/harder than it looks. Get some appropriate copper stock (16 Mil thickness would be perfect and can be formed by hand easily enough), cut it to the appropriate width and length (long enough to at least slightly overlap when in place, trimming to length as necessary), form and wrap it around the lams and coil form so that the ends come together under the transformer, solder the ends together (it will take at least a 140 watt gun or more), reassemble, paint (as desired), reinstall, and reconnect.
It would be a worthwhile effort for your restoration project, as I would imagine that the unit will not meet its hum and noise specification until the strap is reinstalled. It may be sufficiently quiet without it depending on your personal preferences -- but doubtful that it would make spec. The SA-300(B) amplifiers are rather sensitive, requiring only 0.90 vac to produce full power output when the level controls are advanced to full.
Good luck with it!
Dave
It would be a worthwhile effort for your restoration project, as I would imagine that the unit will not meet its hum and noise specification until the strap is reinstalled. It may be sufficiently quiet without it depending on your personal preferences -- but doubtful that it would make spec. The SA-300(B) amplifiers are rather sensitive, requiring only 0.90 vac to produce full power output when the level controls are advanced to full.
Good luck with it!
Dave
Yes, I quite agree. That transformer must have had the strap in place when it left the factory.
It is difficult to imagine why someone would remove the strap. Maybe the transformer was repaired at some point and whoever reassembled it could not fit the strap back in place. We will never know.
Thank you very much for the information/instructions for replacing the strap. My first step is to get the new capacitors in place and make sure everything is running as it should, then, I may tackle the copper strap issue.
Sorry guys....
The amp in storage is another SA300B.
Tcdriver....maybe you can snap some pics of the SA300 and SA300B you have on your bench side by side.
gullaz
Active Member
I've always assumed the sa300 has a higher b+ than the 300B : Sams schematics reports 400 to the el34 anodes for the first while the Fisher manual for 300B states 370/380v (which is more or less what I measure in mine). Though I never had a chance to measure a 300 and Sams is not always accurate. If confirmed this would account for a slight output power difference between the two.
Interesting that on my very early SA-300 (Serial #10446A) the Balance switch is mounted on the rear panel next to the auxiliary AC outlet, but on your SA-300, it is on the front panel. Don't know what the serial number of your SA-300 is, or what the serial number change over was to the B version (normally would have occurred at an interval of 10000), but it may be that the relocation didn't have anything to do with the B version officially, but was simply a production change made at some point during production of the original SA-300.
The main B+ voltage in my unit is spot on with the Sams voltage indication, but that also is when operating from the typical AC line voltage in my home of 121-122 vac. From a 115-117 vac line (more in line with what the unit is rated to operate from) my B+ voltages would have been closer to Fisher's. That indicates that the Sam's B+ voltage is on the high side since their voltage readings are based on the unit operating from 117 vac.
The published increase in rated power output was for the B unit -- but that was without doubt just a smoke and mirror game since the new IHFM ratings came out during the production run of this model. Fisher likely took advantage of it (like they did in their receiver line) to market the B model better again the 35 watt/channel offering from Dynaco. The power output capability between the original and B versions of the SA-300 is almost certainly identical.
Dave
The main B+ voltage in my unit is spot on with the Sams voltage indication, but that also is when operating from the typical AC line voltage in my home of 121-122 vac. From a 115-117 vac line (more in line with what the unit is rated to operate from) my B+ voltages would have been closer to Fisher's. That indicates that the Sam's B+ voltage is on the high side since their voltage readings are based on the unit operating from 117 vac.
The published increase in rated power output was for the B unit -- but that was without doubt just a smoke and mirror game since the new IHFM ratings came out during the production run of this model. Fisher likely took advantage of it (like they did in their receiver line) to market the B model better again the 35 watt/channel offering from Dynaco. The power output capability between the original and B versions of the SA-300 is almost certainly identical.
Dave
Below is a plot of my SA-300 B+ Voltage VS Line Input Voltage. Since I installed a CL-80 the actual voltage reaching the primary winding is about 2V lower. That 2V lower includes a voltage drop across the CL-80 of about 1.5V and 0.5V across the fuse. As one might expect, the power output drops as the line voltage is decreased. Power output measurements indicate almost identical power output between my SA-300 and SA-300-B amplifiers. I believe the power difference is solely due to the "IHFM" power ratings.
The serial number on my Fisher SA-300 is 22748C. In looking at the advertisements in High Fidelity Magazine, for the SA-300, one can see that the Balance switch is located, as you describe, on the rear panel next to the auxiliary AC outlet.
