I have a few tape decks and am tempted to further the mods I have made to some of them by changing the input transistors on the playback circuits. I have attached schematics and was wondering what people thought to this suggestion, my main aim is to improve SNR and playback performance/linearity. I will be upgrading the signal path capacitors and The "best" input section I have seen is on my Pioneer CT-91 and Yamaha KX-1200 decks which use the 2SK389 dual FET.
I have a reasonable number of 2sa1085/2sc2547, 2sk369 and other low noise transistors
1. Akai GX-77, uses 2SC1844 and 2SC1312. Replace with 2SC1845 or 2sc2547? The noise specs on the 1845 seem slightly better than the stock 1844.
2. Revox A77, uses BC109, any ides what I can use here? noise figure I have found is 4dB for the BC109's
3. Nakamichi 682ZX, uses 2SA970, replace with 2SA1085?
4. Nakamichi Dragon, uses 2SK170, replace with 2SK389?
5. Akai GX-280, uses 2SC871/2SC458, replace with?
6. Tandberg TCD-440, uses BC651/BC549C
I have a reasonable number of 2sa1085/2sc2547, 2sk369 and other low noise transistors
1. Akai GX-77, uses 2SC1844 and 2SC1312. Replace with 2SC1845 or 2sc2547? The noise specs on the 1845 seem slightly better than the stock 1844.
2. Revox A77, uses BC109, any ides what I can use here? noise figure I have found is 4dB for the BC109's
3. Nakamichi 682ZX, uses 2SA970, replace with 2SA1085?
4. Nakamichi Dragon, uses 2SK170, replace with 2SK389?
5. Akai GX-280, uses 2SC871/2SC458, replace with?
6. Tandberg TCD-440, uses BC651/BC549C
Attachments
Here are my selection criteria:
- Use metal case transistors, not plastic
- BCY58/59 (or BC108/109, BC549/550) is fine, I have a large stock of BCY58 group X for general use
- Use the C version if there is A, B, C grouping. Use the IX or X version if there is VII, VIII, IX, X grouping
- If you have a handheld multimeter with hfe (β) measuring capability, chose the highest β
- At one point the tape hiss dominates anyway
- Use metal case transistors, not plastic
- BCY58/59 (or BC108/109, BC549/550) is fine, I have a large stock of BCY58 group X for general use
- Use the C version if there is A, B, C grouping. Use the IX or X version if there is VII, VIII, IX, X grouping
- If you have a handheld multimeter with hfe (β) measuring capability, chose the highest β
- At one point the tape hiss dominates anyway
Most of those transistors are quite low noise for the purpose as it is, as tape noise will dominate anyway. 2SK389 is a dual 2SK170, which is low noise in itself.
Sorry for the Dragon I meant replace the 2SK170 with 2SK369. 2SK389 is a dual 2SK369 I believe, and the 369 noise level is about half that of the 170.
I agree that the SNR of the two Nakamichi decks can't be improved by much, but I think all of the R2R's can be, given they are generally older designs (the Revox is late 60's) and theoretically the SNR of the reel to reel is greater than that of cassette given the track width and speed. The best I SNR I have seen quoted anywhere for cassette is 62dB, so 65dB should be acheivable for 3.75ips R2R and 68dB for 7.5 ips, surely? The specs on the Akai GX-77 is 63dB SNR (I guess at 7.5 ips, speed not quoted) and the other R2R decks are worse.
I agree that the SNR of the two Nakamichi decks can't be improved by much, but I think all of the R2R's can be, given they are generally older designs (the Revox is late 60's) and theoretically the SNR of the reel to reel is greater than that of cassette given the track width and speed. The best I SNR I have seen quoted anywhere for cassette is 62dB, so 65dB should be acheivable for 3.75ips R2R and 68dB for 7.5 ips, surely? The specs on the Akai GX-77 is 63dB SNR (I guess at 7.5 ips, speed not quoted) and the other R2R decks are worse.
i'd have to check the datasheet to be sure but I think the 2SK369 has much more transconductance and significantly more capacitance. those differences might warrant consideration regarding impacts of your changes to frequency response and distortion.
mlloyd1
mlloyd1
Thanks for the advice, I might leave the 2SK170's as they are then.
With regard to the NPN's, If using the 2SA1085/2SC2547 will it just be a case of adjusting the collector resistor to adjust the operating current? According to the datasheet, the 2547 seems to have the lowest noise at around 0.5mA, and the 2SC1845 at around 0.2mA.
With regard to the NPN's, If using the 2SA1085/2SC2547 will it just be a case of adjusting the collector resistor to adjust the operating current? According to the datasheet, the 2547 seems to have the lowest noise at around 0.5mA, and the 2SC1845 at around 0.2mA.
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