[HiFi] ROTEL RA980BX ερωτηση για ''fine adjustement''

[east electronics]

O συγκεκριμενος ενισχυτης εχει τοπολογια ''διαφορικο διαμαντι" οπου ειναι απολυτα λογικο οταν εχεις τοσα ενεργα εξαρτηματα στην εισοδο να εχει καποιο μικρο οφφσετ . Το επιπεδο που εχεις ειναι πολυ αποδεκτο και μην σε ανησυχει καθολου Γενικα δεν υπαρχει απολυτως κανενας λογος να επεμβεις εκει .

Δυο πραγματα που πρεπει να κανεις σε αυτο το μηχανημα ειναι να αλλαξεις τον ρελε της εξοδου γιατι εκει εχει κανει μαλλον τσιγκουνια ο κατασκευαστης .

Στατιστικα οι ενισχυτες αυτοι δεν παθαινουν απολυτως τιποτα ειναι πολυ σωστα κατασκευασμενοι εκτος απο τις γεφυρες ανορθωσης οι οποιες βραχυκυκλωνουν χωρις σοβαρη αιτια ...και εκει μαλλον η εταιρια ψωνισε απο λαθος μαγαζι ...

Μπορεις να τις αλλαξεις με κατι μεγαλυτερο στην ιδια θεση για να εισαι σιγουρος για το μελον .

Ασφαλεια για τις γεφυρες θα ηταν επισης να προσθεσεις bleed resistors τυπου 3Κ3 2W πανω στους πυκνωτες τροφοδοσιας που φροντιζουν για εξομαλυσνη του φορτιου αλλα και ταχεια εκφορτιση κατα το σβυσιμο ( ειναι απαραιτητο )

Εαν μπεις στην διαδικασια υπαρχουν αρκετα ψιλολογια τα οποια μπορεις να αναβαθμισεις τα οποια θα εχουν σαφως καλυτερο ηχητικο αποτελεσμα .

Σε οτι αφορα την πολωση αν την ανοιξεις τυπου +30% αναμενεις την ποιοτητα του ηχου σε χαμηλες και μονον εντασεις να αλλαξει προς το καλυτερο αλλα συναμα θα ανεβει και η θερμοκρασια .>>Σαφως με τις πολωσεις τσιμπημενες δεν κανεις παρτυ με αυτο το μηχανημα

Φιλικα
Σακης Πετροπουλος

[peris!]

Μπραβο κυριε Σακη!μου εδωσες ωραια δουλιτσα για το απογευμα!!!εριξα και εγω λιγο ψαξιμο και βρηκα αυτο(προφανως το γνωριζεις...διχνει καλο σαν ιδεα η βελτιωση του προενισχυτη και σκεφτομαι να το κανω) Tweaking the Rotel RA-980BX Integrated Amplifier for Less Noise
The RA-980BX is an upper middle class integrated amplifier from the late '80s that was and still is rather powerful and sports a good phono section. The only major flaw is a fairly high level of noise present at all volume settings. There is something that can be done about this, however.
As we learned in the last entry, the noise (hiss) floor at low volume levels is dominated by voltage noise of the amplifier stage directly after the volume potentiometer. Inspecting the schematic reveals a circuit using an AD712JN opamp here, a FET input device cited to have high slew rate and low distortion – and a voltage noise density of a whopping 18 nV/√(Hz) (where very low-noise parts can have less than one tenth). Adding the noise from the gain setting resistors, we're at a nominal 18.7 nV/√(Hz).
Total voltage gain after the volume pot comes out as a fairly standard 45 dB (the first 16 of which are achieved by the opamp circuit), which gives a computed noise level of almost 500 ΅Vrms at the output, about in line with the spec. This is not particularly grand. Even a hi-fi speaker of moderate sensitivity (85 dB SPL / 1 W / 1 m, 4 ohm) would give off about 13 dB SPL of hiss. From personal experience I know that even 6 dB less still is easily audible.
Now let's improve the noise level. This basically takes two steps:

1. Select an opamp with low noise that fits the working environment.
2. Reduce noise from gain setting resistors.

So let's pick an opamp:

• The new opamp would need to run on nominal +/-18 V supplies, measured to be about +17.6 and -17.3 V.
• It has to be in DIP form factor.
• Source impedance for the non-inverting circuit can be anything between about 150 ohms and 25 kOhm, but typically would not be expected to be more than a few kOhm at normal volume.
• DC offset is not an issue, plenty of bipolar coupling caps here.
• There are resistors to provide input bias current, too.
• Maximum expected output amplitude is only about 1 Vrms, with a load that's typically well in the kOhm range even with the tone controls active.
• Speaking of which, the dual opamps are used in a slightly unusual fashion here, with one half after the volume control and the other as a gain stage after the tone control. The latter sees source impedances in the two-digit kOhm range, unfortunately, thus possibly prompting use of a FET input part in the first place. However, you can always bypass the whole affair, so I'd see this as less critical.

The first guy I discussed this with bravely went with a LM4562 for IC501 and IC502; hopefully the slightly over-spec supplies won't give any trouble in the long run. The closely related LME49860 would be a better choice, it is rated for +/-22 rather than only +/-17 V. Otherwise the fairly low voltage noise (2.7 nV/√(Hz) nominal), good output drive capability, excellent transfer linearity and reasonably low common-mode distortion make this part a good choice here.
The original gain setting resistors are 10k and 1.8k parts (R505/507, R506/50. We can cut these values by a factor of 10 to reduce noise even further, as their contribution would be dominant at low volumes otherwise. New values are 1k and 180 ohms, respectively. Expected total voltage noise would be 2.8 nV/√(Hz) with the volume turned down, for a noise reduction of up to 16.5 dB (possibly limited by the power amplifier and very likely to be less when the tone controls are active).
The proud owner of the modified amplifier stated that the noise was cut significantly. Where it had been plainly audible before, you now had to listen with your ear close to the speaker to pick up any at all. I consider that a success. Subjectively, sound after the mod was perceived to be "sharper, more accurate". (Audible noise is reputed to make sound "softer", so that would fit right in.)

[east electronics]

αν θα μπεις στην διαδικασια να τον ""ψαξεις "" εχει πολλα αλλα ψιλολογια ακομα ...αν θες μπορω να ξεθαψω απο οσους εχω φτιαξει και να σου πω τι αλλαγες εχω κανει και γιατι


Φιλικα
Σακης Πετροπουλος

[peris!]

Μ'εφτιαξες παλι!αλλα και εγω ομως εκτιμω την βοηθεια σου και ανταποδιδω με τον τροπο μου!